3 entries by air, usually not a problem... and tourist visas can be denied at the border anyway... only non-immigrant visas are considered safe. I had 4 entries between Jun and Dec 2019, 3 by air and one by land - never any problems.
As far as I am aware, having a tourist visa does not guarantee entry either... you can still be denied entry upon arrival... a non-immigrant visa is much more secure.
If you're flying they generally ask for proof of onward travell at the airport check in. They want it to be within 30 days for visa exempt status because immigration does not have to grant an extension of stay. In Japan, they made me buy an extra flight before they allowed boarding to Bangkok.
Depends on which extension and which immigration office. Koh Phangan requires a flight HOME to recieve covid extensions now. But not for regular extensions.