ds1 had about 80 words by 20 months (sad, I know, but I wrote them down at the time); ds2 was about 60 words by the same age, dd is now 19 months and only says 'daddy', 'no' and 'get down'. She understands everything though, and will fetch her shoes if you ask her to/put things in the bin when asked/gie kisses when asked and give things to her brothers when asked. She sings away in her baby babble all the time, and does the actions to nursery rhymes, which I don't remeber the boys doing. I'm not that worried about it, just curious as to whether anyone else has had a similar experience with their third.
My ds said only about 4 words at 19 months. However, at about 19 1/2 months a switch must have turned in his brain, and his language development went full steam ahead. On the first day of the "transformation" he spoke ten new words and since then he has been adding a few more each day. He spoke much later than his older sister, but he has added new words to his vocabularly much quicker than she did.
My 3rd is now 3 and is still a bit slower than the other two were, but is doing better now the older two are both in school and he goes to preschool twice a week. I got a referral to speech therapy and she is just monitoring for the moment, and feels he is borderline normal and will improve with his peers.