If it's slipping ...ie tricky to get into 3rd could be worn frictions plates which means a rebuild.   check your fluid to see if its looks or smells burned.  Pull the pan see if any burned friction material is in the bottom.  While your there and if all looks good I would rebuild the valve body. If your lucky its just a sticking valve.  I don't think its the vacuum modulator if its shifting good from 1st to 2nd. But you can always try to replace that first. while you have it disconnected start the engine make sure you have vacuum.