The new combined service will be 208/120v. ...
Will you be changing the 120/240 1? 3W service over to 208/120 3? 4W or simply keeping it as is for the most part and making it 120/208 1? 3W?
If changing over, you can re-balance the sub, and divide the load across all three line conductors.
If not, you will have an unbalanced condition as previously mention, and will have to compensate.
A word of caution on the existing split phase wiring. Any derating of the existing wiring likely considered the neutral as a non-current-carrying conductor. With 120/208 1? 3W feeders and branch circuits, the neutral will be a current carrying conductor. You will have to re-evaluate split phase wiring for possible derating errors. Feeder neutral sizing should be okay, but I suggest you re-evaluate these too.
Calculations
48 + 66.8 = 114.8kW
48 + 66.8 + 64.4 = 179.2kW
Combining without changing 1? to 3?:
114.8kW ? 208V ? 240V ? 0.8pf = 124.4kVA ... ? (208 ? 1.732) = 345.2A [208/120 3? 4W]
64.4kW ? 208 ? 240V ? 0.8pf = 335.4kVA ... ? 208 = 335.4A [120/208 1? 3W]
345.2 + 335.4 =
680.6A on two lines
Combining with changing 1? to 3?:
179.2kW ? 208V ? 240V ? 0.8pf = 194.1kVA ... ? (208 ? 1.732) =
538.9A [208/120 3? 4W]
Don't forget the ampacity of the service conductors must be at least 125% of the calculated values.