I know this is an old question, but I had the same problem and solved it by connecting a diplexer between the "In" from the cable TV supplier and the "in" on the 3-way splitter. This creates a "POE" filter for the Moca signal and "reflects" it back through the splitter. The way the diplexer is connected is crucial, though.
You must connect the cable from the cable TV provider to the lower frequency connector (TV/Ant/Cable/etc) of the diplexer, the high frequency/SAT connector on the same side should have nothing connected to it (or it could be terminated), and the single connector on the opposite side should feed the input on the 3-way splitter.
I hope that was easy to follow
and I hope it works (if you're still having problems).