Can a Belgian citizen re-enter Thailand twice by land for visa exemption after traveling to Cambodia and Laos?

You'll be fine

Just dont use the Poi Pet border crossing - very bad reputation for problems.

Why go to Laos when u can just pay $1700 baht or us $50 n apply for TM7 at any immigrationn office in thailand to get a 30 days extension after u hve used up your 90 days stay

you will not have a problem doing three visa-exempt entries, because you do not stretch each stay to the maximum and you spend time in a neighbouring country - which makes you a "real" tourist in the eyes of the Immigration officer you are standing in front of. Just make sure that you have the THREE proofs: you need to carry THB 20.000 in cash (or this amount in another currency like Euros), you need a proof of accomodation for the next few nights, and you might be asked for a proof of onward travel within the 60 days you will get stamped in