head to head
Neither is simply harder - they are hard in different places. Mega Man X3 demands more execution; Red Dead Redemption 2 demands more variance.

Execution · Practiced Mastery
Tight platforming execution against fixed boss patterns with a solvable upgrade route.

Execution · Practiced Mastery
A cinematic open-world western leaning on shooting feel and atmosphere.
Mega Man X3 leans Execution (Practiced Mastery); Red Dead Redemption 2 leans Execution (Practiced Mastery).
Mega Man X3 leans clearly harder on execution.
Mega Man X3
Dash-jump precision and boss dodging demand tight inputs
Red Dead Redemption 2
Gunplay and Dead Eye reward aim and timing.
Red Dead Redemption 2 leans a little harder on reads, though the gap is small.
Mega Man X3
Mavericks cycle fixed attack patterns
Red Dead Redemption 2
Enemies use scripted AI patterns, not adaptation.
Red Dead Redemption 2 leans clearly harder on variance.
Mega Man X3
Nothing random - every pickup and enemy is placed
Red Dead Redemption 2
Some hunting and weather randomness, lightly hidden.
Mega Man X3 leans clearly harder on optimization.
Mega Man X3
Weapon-order and upgrade routing is a known solved sequence
Red Dead Redemption 2
Loadouts and routes solvable but loose.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.