Right next to the ONT, Verizon installed a 1 in 3 out splitter. One of the outs goes to the router and the other two go to the TV's in my house.
I am going to be calling and getting the free DCT-700 and was wondering if I need to replace that 1 in/3 out with a 1 in/4 out and run the new coax the entire length or can I split it upstairs and just run it to my third TV from there?
If I do need to get a new 1 in/4 out splitter, is there any special/preferred kind I should be looking for?
It depends. The three-way splitter is not really a three-way splitter. It's a 2-way splitter with one of the two streams immediately (internally) split again for three outputs. If you look closely at the 3-way splitter, you will see that ONE output has 3.5db attenuation, while the other two have 7.0 db attenuation. If the line you contemplate splitting is the one that has only been attenuated by 3 db, then you can install a 2-way splitter anywhere in that path. If on the other hand you put another splitter on one of the lines that are already reduced by 7.0 db, you're really suffering a lot of signal loss (10.5 db which is about 90%). You can replace the 3-way with a 4-way, but then you have to probably run a much greater length of coax than if you use a 2-way splitter somewhere downstream.
My recommendation: Ensure that the line you contemplate splitting is the one connected to the 3.5 db attenuated output of the3-way (you can switch those 3 around as necessary to get it set up that way) and then use another 2-way closer to where your 2 TVs/STB's are.
A caution: any splitter or pass through connector you use MUST be capable of passing up to 1GHz. Lots of older splitters are only rated up to 600 or 900 MHz.