Can I mount 3 blades on a 5 blade ceiling fan?
No, it will not be OK.

A fan blade system is meant to be balanced. Even when all of the original equipment blades are in place, minor imbalances can occur and need to be corrected with weights from a balance kit. Failure to have a balanced fan causes wobble. If the wobble is bad enough, it can damage the motor, loosen the fan in its mounting and even cause mounting failure.

The five existing blades are evenly spaced. You can get replacement blades, but they have to be the same number as the originals. If you mount on only three of the five brackets, two blades will be next to each other and one will be opposite. This is a very significant imbalance and could lead to catastrophe.

If there were a way to mount the blades evenly around the perimeter, it might work, but that would require a whole new and custom crafted bracket system.

SUPPLEMENT: The Questioner has added information, now indicating he intends to mount the blades evenly spaced. If that is achieved, balance should not be an issue. Care must be taken to ensure that the adapter used to mount is also balanced around the perimeter (an added bracket on only one side would still create imbalance).