Good evening.
We have some confusion already.
Thats normal and ok.
Now lets answer the best I can and please remember, as this is not a book, my answers will be in a general view of the subject of Trout Spey type rods.
Q1 the rod shown is a slow action, because it bends along the entire length.
Correct.
The reason for this is the caster will only be forming a D Loop behind him or her self when loading the rod (with the aid of the anchor effect… more about this in Casting )and NOT and not by an airel load (the line in the sky) as in overhead casting like so called single handed rod casting, that most people perform, which will require a fast rod to max, the potential of the load to the cast distance.
Today, there are few slow actioned rods available on the market, if any, why, there are many reasons one of which few people can Spey cast with their single handed fly rods, INCLUDING the so called experts that work for and advise some Companys.
Hence no demand for a product which equals no possible sales which equals no investment in this class of rod, in single handed fly rods.
The end result consumers cannot fish anywhere, like a spin fisher can, all because of the need of a back cast area, which most Spey casts do not need therefore where a spin fisher can fish, so can a Spey casting fisher with either a single or Double handed rod.
And now to confuse most people.
You can Spey cast with a fast rod !
However the ideal, the best performing rod for the task is a fly rod designed for the cast, the task of Spey casting, which is a slow actioned rod that bends all the way into the grip.
Q2, It is a 10’ 3wt full flex blank.
Beautiful !
Yes you can build a Switch rod or a Sweet Trout Spey from that blank.
Switch rods have a shorter top grip, and faster action than a Spey rod, in the ideal World.
Anything under about 11 feet you start to lose the effect of the loading from the D loop that is formed in a Spey cast.
You can over come this with a little more casting skill AND SPEY Casting is far easier that you think !
It will seem clearer or easier if people start to think of this D loop term as the power loop, that drives the energy in and for the cast. The people who I have instructed appear to accept the facts and understand some what quicker, when this description is used.
Back to the 3w 10footer case,
Why not make a little Trout Spey with a 3w 10footer, because there are no concrete rules in the fun department.
AND a 3w TS will be fun 
a great little rod that you will be able to single hand cast with also.
BUT…you guessed it, could this be classed as a Switch rod also.
The answer is yes and no.
Yes, because you can cast with both single or both hands.
No, because its a slow actioned rod.
A kit to build from,
just buy 1 guide for every rod length foot, (you can use less if you chose) buy a reel seat, anything you like, as per a normal single handed rod, as long as both ends are open
With a big number one stripper guide, say about 3/4inch ID.
grips, cork, bottom, something about 4 1/2inches to 5 inch.
Top, the ideal length will be, from the bottom grip, to the bottom of, to what ever your shoulder width is plus 2 inches.
However with a little 10 footer 3w, this could then be say 14inches, as the rod will not need to be driven hard at all.
Q3, do I need to buy a special 6wt “SPEY” fly line?
NO, unless you have money to burn !
And in the case of a suitable line for this new 3w TS, I don’t think there is even a 3w Spey line made.
So we will adapt and build a line…thats latter… more fun and exploring, because we can now introduce a number of different tips to the line, say, a floater, slow sink, intermediate and a fast sinking AND carry all these tips in your shirt pocket.
Q4, S Lines, Aren’t they longer…
Yes, because generally a Spey cast is further than the single handed casts, eg, the World Record Spey cast is something like 220 feet !
Q5, and built differently for spey casting?
Yes, in most cases the front taper is longer and the belly is longer,
there are 3 belly lengths built, 50/55feet, 65’ish, 75’ish feet in length.
With your about to be built wee little TS, I would suggest to start with a belly in the range of about 3 to 4 times your rod’s belly length, plus then a forward taper, more on the subject latter.
Or we can adapt a single handed fly line to use your casting and load as required.
Q6, does that mean I need a larger reel to hold it?
Yes, it does, normally it is a case of increasing the reel size by two for a single handed rod size, to allow for the longer forward taper and belly lengths.
In your case a 6 or 7w reel will be ideal.
To clear away some fog,
Some will have noticed, that some Doubles, in all types are rated as, 5/6 or 7/8w or what ever, in two line sizes.
these indicate, to some ( its debatable to some) the lower size/number is the tip size/load and the higher/larger weight number the butt load.
There you can cast a full butt load or a lighter tip load or say for example, a tip load and then add a heavy sink tip and cast a full butt load. (bet someone is confused by now )
And now some ideas on line size loads for different rods,
example a 6w
Single Handed rod, line weight should be about 160 grains.
Double Handed loads,
Shooting Heads, 250 grains,
Short Spey belly lines, about 50 feet length, 420 grains
Mid S bellys, 450/460grains
Long S bellys 600 grains
Why the increase in size over a overhead casting single handed rod,
because you do not progressively load the rod with the line in the sky, eg, false cast, and false cast a number of times till there is enough line load to shoot the line and fly to the selected loaction.
The Double will be loaded from the D loop.
The Double when useing a Shooting Head will only have the Head outside the tip before the final cast stroke.
Enough for now, a little more if requied tomorrow maybe.
Remember, there are no silly questions, just way of learning, the easy way.
Kind regards,
UB