Total noob question here, but I’ve been trying to use a third sheet as a source and I cant see the third sheet.
I am using a the free version… would this not be the case if I went with the pro app?
I’m not clear on what you’re trying to ask here. Do you mean you have 3 sheets in your Spreadsheet and can’t see the third one in Glide? Do you have headers in that Sheet?
No headers in the sheet
Thank you 
You should have at least 1 line in the Sheet for Glide to be able to read it. The first line will be considered as your header.