Which form do I need for this? The TM47 or the TM7 or both?

Both (if you have already extended) and 1,900 baht for the TM.7 - if it is the first extension of a 90 day entry your 1st TM.47 will be due 90 days after this application made as first extension counts as a 90 day address report.

it's my 3rd extension

for the 2nd one, I thought I didnt need to do a TM7 again, but they made me do one

The TM.7 is the extension - if you want an extension you must use that form and pay the fee. Only if you received more than 90 days on your permitted to stay would you just visit to report address with the TM.47 form.