One day at work during a cold spell, I let my car warm up then went home. But it wouldn't shift into 3rd. The previous day it was colder and it worked perfectly. There was no symptoms of tranny failure. So next day I tested it but this time I didn't let it warm up. It shifted fine till it got to operating temp. But it didn't drop gears i stopped at a stop sign and it just wouldn't shift into 3rd. Could it be a shift solenoid or the speed sensor?

dont know much about autos but it would help if you put the year model and trim. or you could fill out the sig.

95 Buick skylark custom 2.3 clyinder

When was the last time u serviced the transmission fluid and filter? How many miles ago?
The place to start is check with the car running on level surface, in park after 20 minutes drive time and ensure you have clean fluid red, and at the full line.
DexronVI (6) on your next service may help if it is low on fluid, otherwise a transmission scan to obtain any stored codes would be the next check.

I changed filter and fluid less about 3k miles ago fluid is good color and smell. As well as in good range.

hmmm...
Either sticking pressure solenoid or accumulator 2-3 valve sticking from varnish build ups..?
Trans Tune - Sea Foam Sales Company
TransTune removes varnish is safe to add to your trans fluid and may greatly improve shifting characteristic quickly. Try adding a half a can or full can if ya got room, to start with...
and get transmission scanned for codes...!

Oh & Which brand/type DEX__? Transmission fluid did u use when u changed it last?

What ever was recomended. There was no shavings on the magnet either

Did u do this job yourself oh had shop do it?
Aligning the filter and removing the o-ring and installing a new one correctly is part of a good install too. Bad o ring install or none= problems.
Always use a gm/delco filter & gasket kit too.
Dexron 6 is the newest synthetic fluid, and transmissions work and perform better with it.
You serviced the transmission 3k miles ago, how many miles were on the old fluid?
It is a good idea to do the filter and fluid every 50k miles in the modern electronic transmissions, regardless of what they say. The friction modifiers do break down and heat is the transmissions enemy. I also add this to my transmission fluid which keeps it cooler and smoother, extend life of fluid too!
LUBEGARD Automatic Transmission Fluid Protectant

A shop, and I don't know when it was last changed i changed it at 110k miles. I did it only because I didn't know if it was done. I don't think it'* a bad tranny I thinking a sensor like a vss, or shift solenoid