Im 13 weeks pregnant and was prescribed cephalexin 500 3x/day for a uti. is this the appropriate dosage? ive been reading it should only be taken twice a day?

It is not unusual during pregnancy to go with a higher dose of some antibiotics because your immune system is "dumbed down" so your body doesn't reject the baby as a foreign object. The body can't help as much to fight infections. The dose you describe is by no means out of the usual. At times, taking more frequently helps keep blood level from dipping too low between doses.