No one makes an evenly split 1:3 spitter for reasonable prices. Every consumer-priced 1:3 splitter is internally a cascade of two 1:2 splitters. So one output gets 50% of the signal (3 dB of splitter loss, plus 0.5 dB of additional insertion loss from the internal components, for 3.5 dB of loss all told), and the other two outputs each get 25% of the signal (really 7 dB of total loss).
It's best to use splitters that only have as many output legs as you currently need.
Leaving one or more output legs disconnected does not decrease the splitter/insertion loss experienced by the other output legs. In fact some signal will reflect off the unterminated dead ends and degrade the signal. The split second it takes to travel down the dead end and reflect back out makes it out of sync with the rest of the signal, turning it into noise / interference. You can buy a 75 ohm f-type terminator to cap off the unused output leg and absorb the signal instead of reflecting it, but again, it doesn't reduce the signal loss of having a splitter between your source and sink. The terminator cap contains a 75 ohm resistor to absorb the signal (effectively just dissipating it as a tiny amount of heat) so it can't reflect back into the system and interfere.
