what sort of cable should I use to add extra tv outlets
Well, if you are going to split it 3-ways, buy a 3-way splitter not a 4-way.  I agree with the comment from Mikes about buying a more expensive model if possible.

I would avoid a booster at all costs - power boosters add noise to the line - so yes you get a higher signal, but its noisier.

As far as minimizing signal loss, here are the top items to keep in mind:


Keep runs as short as possible
Don't splice cables together - always use a single continuous run (where possible)
Buy a high-quality RG6 cable.  Quad Shield (usually you will see RG6-QS on the cable) provides the most shielding and helps reduce interference
Use high-quality compression fittings instead of crimped connectors (Thomas &amp; Betts Snap-n-seal is my product of choice).  Also make sure you install them correctly (end of internal plastic insulator flush with the inside of the fitting, ground braid properly folded back, cable stripped correctly, no nicks in wire, etc.).