I am trying to replace one of two ceiling fixtures that are controlled by a switch on the wall.
My problem is that I have 3 black wires and 3 white wires and don't remember which one connects to which one.

First turn the power off and start testing your wires with a pig-tail light, usually with that many wire ,one is the feed and the other two run to the other lights.

How many switches control the light? These are lights only, not fan/light combinations right? How many wires in the switch box? Do you have a meter?

If the switch, does NOT use a white, then this will be easier.
Then you could likely connect All whites(unless you remember different)
Then you are left with 3 blacks. 1 of those hooks to the light, the other 2 connect together.
You could identify each black as: 1, 2, and 3, or black, red and blue tape. Then their would be 3 possibilities, black and red identified connected together, blue to light. Then try red to light, with black and blue together, one last posibility would be black to light, with red and blue tied together.
If the switch controlles 2 light boxes(fixtures) it will need different method.