Seedless watermelon, with 3 seeds? can I plant those 3 and get....?

Not likely. You may not even get anything edible. The genetics of the seedless variety don't carry over well from the commercial fruit. Go buy some good seeds at the store if you really want to plant watermelons.

Seedless watermelon variety is produced by the cross between a tetraploid and a diploid to get the triploid seeds. These triploids cannot produce the seeds. The seeds which you got in a seedless (?) watermelon fruit are produced accidently in this fruit. We dont know its genetical structure so we cannot predict the nature of the fruits whether they will be produced or not etc.

I think the odds will be 50/50

Yes, you will have a crop of seedless of your own

probably

of course