As a general rule, heartworm pills contain a product known as Ivermectin. The amount in each pill is a very small dose and usually isn't enough to cause a big problem unless it's overdosed.
Since your dog only ate 3 more than he should have, I don't think that you have anything to worry about. If it had been a 12 pound poodle who ate 4 heartworm pills for an 80 pound dog, I'd be worried. But since I assume your Lab is a bigger dog, there shouldn't really be an issue.
That being said, you should still keep your eyes open for signs of Ivermectin toxicity. Things to watch for would be things like lethargy, anorexia, depression or even vomiting and diarrhea. If you notice any of these problems, you should have him seen by a vet immediately, but I don't think it'll be a problem.
I hope this helps.