If someone says "I second that", is it grammatically correct to say "I third that"?
The reason is because it is a reference to "parliamentary procedure."
Anything to be proposed in a parliamentary situation is:
1. First proposed by motion by one member of the body;
2. Seconded by another (different) member of the body. It doesn't mean that this person necessarily agrees with the first person, simply that this second person wants to see the matter debated or voted on.
There is no need for a third or fourth or fifth vote..... two is all that is required.
So there is no need to "third" anything.... saying that would simply mean that you don't know what the origin of the terms being used are.
Instead. You would simply say something like "I agree" or "Yes we should vote on that issue."
In some legal settings that require motions to not only be seconded but thirded you might say it, but it would sound strange in everyday language. As others have suggested you could say "me three", or some other possibilities are "me too" and "I as well" or "I also support that".
I would simply say "Third!" or "Third it!"
Yes you can say it. There is no grammatical rule that says you can't.
"I agree as well."