I have a 2x3 tube frame for my Model T coupe. Do I need a k member , x member or just a transmission support cross member?

Just a trans crossmember is traditional, but a K member ads a ton of torsional strength.

Try to get a triangle or three into your design. X-member would be great but K is nice second choice. Saving grace is Ts are so light and square tubing frames are so rigid to begin with, a few robust and well installed straight X-members will do just fine.

T Coupe, 23-25, 26-27, 17-22, hiboy, full-fendered with boards and aprons???So many specifics, so little information.
4cyl.automatic, V6, V8, 4speed, 3speed, PowerGlide, small block, big block, ??
9", 8', 10-bolt, 12-bolt, Jag IRS, 'Vette IRS, 55-64 Chevy rear, coil-overs, coil springs, buggy spring, dual springs???

The traditional ladder style frame is fine for most T applications due to limited frame dimensions and space availability.

I have a 2x3 tube frame for my Model T coupe. Do I need a k member , x member or just a transmission support cross member? 350 chev-700r4-rear coil over rear with 9 inch-front leaf with 4 inch drop tube axle-no fenders

A transmission crossmember should do you just fine. If you have the typical kickup at the rear over the rearend and tubular front crossmember like a bucket that'll do the job.