You will not lose 3dB at each connection, maybe 0.5dB max as long as the connector is good (probably more like 0.1dB)
Anyway, just log into the modem diagnostics at 192.168.100.1 and make sure the downstream level is between -8 and +8dB also the upstream needs to be at 50dB or lower. This is where the splinter losses come into play, you can add or remove a split (or rearrange the outlet connections) to get you in the range you need. A 2 way will lower downstream signal by 3.5dB and raise the upstream power level (how high the modem has to amplify the return signal) by 3.5dB also.
Also, depending on the modem, you will see the downstream channel frequency. As long as the splinter is rated higher than that you're good to go. Pretty much all new splinters are fine, some are just built a bit tougher and are shielded better(look for a cast body with a soldered backplate not a glued one)