Clackbridge Troll
Oracle Text
Trample, haste
When Clackbridge Troll enters the battlefield, target opponent creates three 0/1 white goat creature token.
At the beginning of combat on your turn, any opponent may sacrifice a creature. If a player does, tap Clackbridge Troll, you gain 3 life and draw a card.
Why exactly?
Donate it to help your Goat Tribal deck out.
For your last part, are you saying in one turn gain 9 life and draw 3 cards or throughout the whole game? His effect happens once each combat on your turn, unless you have multiple combats tricks/turns.
I did have someone make an incredibly silly play and suicide themselves against me by playing this out while I'm playing a cavalcade of calamity deck in standard.