Yes the 3rd limb can be the main trunk, But it sounds like that's a limb coming from the rootstock (below the graft) and unless you plan to bub/graft it it won't set the grapefruit you bought. Two choices, one, cut it off, two allow it to grow and bud/graft it to the fruit type you want

good news. i didnt mention it was above graft, sorry. ill trim at broken branch scar( that is on 3rd branch to be new main)and let it do what it does . whats wrong with origanl grafted wip , skinny and no growth except for small 2 branches. the root stock is getting thicker. do u agree with my trim plans, and when is best time to put in ground? i now have in a 7 or 10 gal pot. thanks for help, ernie at foothill festival,

The best time to plant was "last year", the second best time to plant is "Now", get it in the place you want so this spring when the root system becomes more active they have a place to go. I don't see any problem with your pruning, just remember, citrus produce fruits on new growth.

it is not uncommon for the first growth (original graft) to not grow well if it was stressed at the nursery and I dare say I can't remember a Wal Mart tree every looking good. I never buy Walmart nursery. that said, not to worry, it sounds like you are getting good growth above the graft. So, make sure there is no growth coming from below the graft and... and I know, this is hard... give the good branches above the graft a good pruning back. Each good branch should be cut back to about 6 or 8 inches (I know, the look of horror at the thought.) The thing is that it will encourage branching and give the young tree better structure for supporting not only the next generation of branches, but also the very heavy fruit. that sort of pruning you don't do every year, but only the first year, and somewhat the second, until the tree starts to get more limbs and some branch structure.
I have over 100 citrus trees and I can say from experience, they will be much better looking and growing trees at the end of '09 if you do.
good luck
Arthur

well grapefruit is doing good lots of new stems and leaves. this semidwarf is growing fast and flowering new leaves are shiny but a lighter green. i had it in a large pot 1 yr after buying as a wip. trunk thicking as well. my myers lemon is giving lots of fruit and flowers but im removing them it needs to leaf out better i think. it on flying dragon stock. grapefruit is on c35. navel orange is on flying dragon. dark green leafs growing slow giving fruit but i remove them. any imput is welcome.