playground 到底有哪几个介词,（on in at)书上似乎都出现过,哪个才是真真正正是正确的,还是都对?

都可以,但是含义各不相同.
1.on the playground是最常用的,强调“在操场的表面上”.
如：The students are palying soccer on the playground.
2.at the playground比较少用,只表示“在操场那个地方”,有时可以和on the playground换用,强调“操场是个小地方”.
3.in the playground强调“在操场表面的地里面”.
如：There is a hole in the playground.
在操场里有一个坑.
注：最常用的是on the playground,除非是强调“在操场地表里面”时用in the playground.