It may or may not be "too late" depending on when your top programs hold their meetings. I know my #1 has already met because I know one of their residents (luckily I sent my email about a week before it).
These questions about letter of intent have been asked and answered multiple times and there is no right answer. The answer usually is "might help, probably won't hurt."
I'm a fellow applicant and I personally only sent out one e-mail to my #1. I think it doesn't make sense to tell 3 programs you are ranking them highly.. if you're not saying they are number 1 then they probably aren't. However, I have heard advice to send "I'm ranking you highly" e-mails to #2 and #3.
In the end, I do not think these letters change anything. I personally did get a nice and encouraging but non-committal response to my #1 e-mail. I don't necessarily think it means anything. Some programs have a rule of not reading them and/or not responding to them. So try not to freak out if you don't hear anything.