Hello Envyme,
It really depends on your usage trends and room size.
By the rule of the thumb, the larger the room size the larger the BTU required to cool down the room (9K,12k,18K etc)
Other factors include morning/afternoon sun, design factors.
The size of the room aside, supposedly with the outdoor unit at total 27K~28K BTU, if you need to turn on all 3x9K at 1 go it can support.
However if your matching is something along the line of 27~28k BTU for outdoor unit and you have matched 1x12K and 2x9K indoor units then turning on all 3 units might affect the cooling efficiency. But if you only use 2 units at 1 time, say 1x12K + 1x9K = 21K or 2x9K = 18K then supposedly no issue to support.
Best person to advise will be your appointed installer.
Give them your room size and also tell them your usage trends during your purchase they will be able to better advise.
Best Regards,
Jason Tham
Not pro in aircon is this enough for the room 9 k?