"Three tomatoes are enough."
Translation:Üç domates yeterli.
The sentence could be "Üç domates yeter." too. But, "yeterli" is an adjective, and "yeter" is a verb. Viz, you can't say "üç domates yeter değil." instead of "üç domates yetmez." (to suffice). But you can say "Üç domates yeterli değil." because it is an adjective -(like "güzel" (beautiful) / "güzel değil" (not beautiful) -). And we use "yeter/yetmez" which is a verb for estimating things if they will be enough or not and "yeterli/yeterli değil" which is an adjective for certain things.
can somoene answer this question? I also want to know why -li is being used
Why is "üç domatesler yeterli", i.e. plural of domates marked wrong? I am grateful for any help.
You don't need to add the plural suffix because of the number before the object. This is due to Turkish grammar, which is the opposite of English grammar in this instance.
Is yetersiz not enough?
Logically, yes. You would normally see "yetmiyor" though :)
Interesting, is that related to the progressive aspect in some way?
Yep, there is a verb yetmek. It is in the present continuous tense.
It is optional and can be used whenever you can physically count something.
Why isn't it tokum, or do you use it only for full (like with eating, " I'm full enough"?)
"tokum" only means "I am full." It has nothing to do with "enough" :)