My 3 yo hallways seems to have darker yellow pee. no pain or other sx. he's potty training in a little toilet so it's obvious. should i worry?

Not diluted: Urine looks different in a potty than in a toilet full of water. It may not actually be any darker than anyone else. If your three year old is happy, eating, and potty training , s/he is likely fine. If you are worried, however. A visit to the pediatrician would be helpful.

Pee: Probably not. He's just a bit behind in his fluid intake. Drink up!!!