We had the same concerns with our nearly 3 y.o. She was definitely a bit freaked out by the panto villain at the start and spent the first half cuddled into either me or my wife. By the interval she was asking to go home , but we had some other kids with us and didn’t want to have to split the group, so managed to convince her to stay with a little lolly-pop bribery.
By the end of the second half, she was singing and dancing with the rest - and even now, nearly a week later, she keeps on telling us all about various scenes that she remembers (she was especially taken with the bit involving the egyptian mummy) to the point of even reciting some of the lines!
As @oakr says, anything outside the Front Stalls (M onwards) is on a good slope so unless you’re really unlucky enough to get a 6 footer like me sat right in front of you, your 3 y.o should be fine. We were on row N and everyone was able to see OK.