The first or third option are both used. The first option is probably considered the most correct. If you're going to use the present tense, I would suggest using the present continuous "I'm just now seeing your email." I actually think the present continuous would be a good way to put it.

1. I just see (in your title) = This is incorrect. The tense doesn't exist. 2. I just seen (in your question) = This is incorrect. Some uneducated native speakers may say this, but it is completely ungrammatical. You should never use this form. 2. I've just seen = This is fine. 3. I just saw = This is fine.

I guess the third option is so cool. But "I've just seen your email now" is right too. The second phrase looks like wrong to me.

Let me add a comment as another native speaker SuKi's answer is correct and the best you received.

I'm not an English Native Speaker but as far as I know, "I've just seen" is usually used by British People and "I just saw" is usually used by American People. Personally I think that in an email, "I've just seen" is more formal :) "I just saw" is a bit conversational