dBs ....
Re splitters (and with apologies for length of explanation):
dBs are on a logarithmic scale:
3dB = 1/2 power
6dB = 1/4 power
9dB = 1/8 power etc
A perfect (theoretical) lossless splitter would give -3dB to each leg of a two output splitter. A practical 2 way splitters has losses of about 3.5dB to each output. Clearly a 3 way splitter can't give -3.5dB (about half power) to each of 3 outputs.
Generally, the lower the loss (3.5 rather than 6 or 7 dB), the better.
By design a (symmetrical) 3 way splitter can give equal outputs (eg -6.2dB in your example). ie only 1/4 of power goes to each of 3 ports (and 1/4 power dissipated in the splitter). In the context of a cable box which seems to be fairly sensitive to level, this is a loss that may well require amplification in order to receive all channels. It's difficult to be sure.
The non-symmetrical splitter (3.5/3.5/7 dB) would allow the two cable boxes to receive the maximum signal BUT would result in a significantly lower signal (-7dB) being available for the internet modem. I have no experience with this but it could be worth a try.
With either option of splitters, it is possible/probable that an inline amplifier (usually quoted as about £9 from Argos) may be required. On balance, of the two options you give, I would probably go for the 3.5/3.5/7 dB splitter.
As an alternative, you can buy cable splitter amplifiers for about £10 with up to 3 outputs. These should allow you to set levels such that the cable boxes have optimal Signal level. For example, from a Hongkong seller there is this item (£7.50 inc postage!!!!) and the seller has >98% with 35000 sales.
h**p://cgi.ebay.co.uk/NEW-CATV-Broadband-Drop-Signal-Booster-Amplifier-220V_W0QQitemZ150107196470QQcategoryZ61395QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
For £7.50, I'd be tempted to have a go for one of these rather than buy a splitter and the maybe have to buy an amplifier as well.
Hope that helps.