1. dictonary form + tokoro desu.
2. te form + iru tokoro desu.
3. ta form + tokoro desu.
the true meaning of tokoro is place, but if joint with others sentence and phrases, the meaning was change, see more at posting before about tokoro desu.
to explain more clearly what the different of each phrases iru tokoro see this picture below
1. dictonary form + tokoro desu.

those picture shown someone prepare to eating, at those time the right expression is
taberu tokoro desu
食べる ところ です
2. te form + iru tokoro desu.

at those picture you see that somebody eating something, this condition is perfect when we use te from + tokoro desu.so the right expression is
tabete iru tokoro desu
食べて いる ところです
3. ta form + tokoro desu.

those picture shown that somebody have just already ate, at this situation the right expression is
tabeta tokoro desu
食べた ところ です。
from explanation in each post about tokoro desu, and this post, we hope this explanation more completely and easy to understanding.