Unfortunately I have not tried this. It's a Motorola modem.
if still interested put 192.168.100.1 into your browser that should give you the modem info.
I mention this because the conventional thinking is to use the 2 way split and use one leg for the modem and the other will feed another power divider for your video needs.
However sometimes that is not practical, if it is in your case I would do that.
if you decide to check your modem diag as wired now you want to look at the up and downstream levels.
The D/S should be about 0dbmv plus or minus 8db.
The U/S should be less then 52dbmv (hopefully it is in the mid to upper 40's)
As you add more attenuation (splitters) that U/S level will increase most modems can push about 55dbmv so adding that extra splitter can cause you to lose your lock.
Also your D/S will decrease (if you are near that -8dbmv level you will not lock up).
After checking your levels if your numbers are near the edge I would do the 2 way split and go from there.
Yes the quality of splitters can vary in my opinion Antronix is a decent brand.