Trikes are at a disadvantage is almost every way:
higher rolling resistance
higher aero drag (compared to fast recumbent bikes; not so bad compared to a rider with a non-aggressive position on an upright bike)
higher weight
more flexible frame (most impact on steep climbs)
Higher rolling resistance is not because of the third wheel. Other than a little extra hub bearing resistance, the third wheel adds no drag in theory because rolling resistance force is equal to Crr*weight. You may have an extra wheel, but that just means the weight is spread around more. I suspect that much (or all) of the additional rolling resistance comes from lateral scrubbing of the paired wheels (especially tadpoles). Imperfect alignment of those paired wheels and imperfect Ackerman compensation in tadpole designs (that scrubs speed with every minute steering correction being made due to pedal steer or even road camber) being the main culprits. And there are a lot of steering corrections, even as you ride straight. Every little bit of unbalanced lateral motion of the legs requires a steering correction and is therefore made plain as day on a trike whereas a bike's ability to freely pivot conceals this. Every correction scrubs speed. This is much more noticeable on a tadpole than on a delta trike because there are two steering wheels on a tadpole. Deltas have only one steered wheel and they seem to have very little steer scrub, just like a bike.