When signals are split in even powers of two (2, 4, 8, 16, etc.), it's pretty easy to end up with balanced output power. You will find that most splitters on the market come with an even power of two ports for exactly this reason.
Code:
+-- out 1
+---+
| +-- out 2
in --+ (all outputs are balanced)
| +-- out 3
+---+
+-- out 4
If you come across a splitter that has an "uncommon" number of ports, e.g., a 3-way split, it will most likely be internally constructed from a two-way splitter followed by another two way splitter. The signal paths that pass through more splitting stages will have a lower signal strength at the output.
Code:
+------ out 1
|
in --+ (some outputs are lower than others)
| +-- out 2
+---+
+-- out 3
So, the short answer is that most splitters you come across will have balanced output power. If you have a splitter that is not splitting into even powers of two, then you might get slight differences in the output power.
If your system is operating close to the edge (where these small power variations actually matter), then it's an indication that you may want a signal amp or some other configuration where the splitter power loss becomes a non-issue.