In theory, 3 spool engines don't need variable geometry since the individual spools could better speed match relative to the 2 spool engines. The original RB211 3 spool engine was "intended" to not need variable vanes or bleeds.
It didn't exactly work out that way
- and by the time it was certified it used pneumatically controlled vane and bleed systems (although the only the first stage of the mid spool had movable vanes - compared to the multiple stages of variable vanes on the same generation 2-spool Pratt JT9D/PW4000 and GE CF6 engines. Theory doesn't always work out in the real world
The current Trent engines have nearly as complex of variable geometry systems as the modern day GE and Pratt engines.
As for 3 spool Rolls engines not experiencing compressor stalls during start, someone forgot to tell the engine
. The RB211 engines on the 747-400, 757, and 767 were far and away the
worst starting engines relative to the 2-spool engines from Pratt and GE, both on the ground and in-flight.