While I am not sure exactly what you are asking, I will try to help.
Generally speaking, your leader (with tippet) will be the same length as the rod that it is going on.
As for size, I'm guessing you are throwing flies in the #12 and smaller range with the 3 wt. rod..... If this assumption is correct, your tippet should taper to what is a 5x,6x,7x, or 8x tippet.
I fish for bass and panfish in kansas city. That being said, I've turned to furled leaders and regular mono or flouro fishing line as the 'tippet'. (some may call 'blasphemy!'to this as it is not a proper tapered leader/tippet setup) On my small rod (2 wt. rod) I tie my own leaders starting with 6lb. test at the fly line, and go down to 4 lb., then 2 lb. at the fly.
5 wt. rod has a furled leader and 4 lb. test flouro 'tippet'.
8 wt. rod gets furled, and 6 or 8 or 10 lb. test depending on what I am hoping for that day. (usually 6 lb. test 'tippet')
If you are going with a tapered leader/ tippet set-up, I'd aim for somewhere in the middle (6x tippet-- should deliver a wide variety of flies adequately-- maybe 7x if you are throwing predominantly smaller flies).
Choosing The Right Tippet Size ? Orvis Fly Fishing
this might help you out.
hope I helped. And I noticed that you recently joined-- welcome. Lots of help here-- I know I hoarde my fair share of the advice here.