Can you define your terms more clearly?
What do you mean by 'split system?' A ductless heat pump with two indoor units?
What does your GC's HVAC guy mean by a 'single outlet?' A ductless heat pump with one indoor unit or something else entirely?
A single head ductless unit will not handle three rooms under any circumstances. You need a register or ductless unit in every room. You can use a single specialized ducted minisplit to heat/cool two rooms--you could handle the bdrm and ensuite this way, but you'll need another unit for the reading room.
You can drive two indoor minisplits from a single outdoor compressor.
Heat pumps produce less heat as outdoor temperatures decline. They must sized to meet your heating requirements at the coldest temperature you expect the heat pump to handle without help from another heater. This switchover temperature is known as balance point. Sizing is a matter of doing a Manual J load calculation at balance point and then using heat pump manufacturer's published heat output ratings at the intended balance point to determine the correct unit size. Oversizing a minisplit for heating will not--in contrast to oversizing a full-sized heat pump--harm cooling performance.
No minisplit will produce useful heat at outdoor temperatures below -4f/-20c. Many will shut down entirely at -14f/-10c. Given Toronto's climate, you must have a supplemental heat source capable of carrying the entire heating load for the addition in cold weather. Use Manual J to size the supplemental heat source.
If you have natural gas, a gas fireplace (for the bdrm) and wall furnace (for the reading room) will work nicely for supplemental heat. If you don't have gas, then look at baseboard heaters, underfloor radiant, or radiant panels.
For the heat pump, look closely at Daikin's multi-head MXS system. These have very good cold weather performance and support both ductless and ducted indoor units. Daikin 4MXS32GVJU is probably adequate for your needs but you need a load calc for a definitive answer.
Sizing minisplits heat pumps for this application is a niche job that few HVAC contractors are able to handle. If your GC's HVAC guy doesn't understand all of the above like the back of his hand, find another HVAC contractor with a strong background working with minisplits.