On my last trip I didn’t have a return as I was planning to get a 30 day extension. However, I got an onward ticket just in case. Just as well I did because the airline wanted proof of my exit from Thailand. It costs about $24 here but it’s worth it for the flexibility it gave me. I like open ended trips but they do cause other difficulties, such as insurance.
Immigration rarely check it but they have the right to and to refuse you entry.
Increasingly, though, is that airlines check it when you check in. Had my tickets checked on two occasions.
A round trip for 3 months isn’t valid because you only get 60 days visa exemption. You can take the chance, of course, and will probably get away with it.