In addition to the reasons stated in the other answers, there is still a bit more on the subject. Often, the number of blades incorporated into a rotor design is about blade/disk loading, load carrying, speed-performance. Many times the number of blades is driven by vibration / ride quality and noise.
In the case of 3 blade systems, rotor vibration and noise have multiples of 3/rev, 6/rev, 9/rev. 2 AND 4 bladed systems have multiples of 2/rev, 4,rev, 8/rev, 12/rev. Odd numbered systems rarely exhibit submultiples great enough to be of concern, while even numbered systems can cover the spectrum. Think 5 vs 6 blades. The 5 bladed MD500s are almost on par with fixed wing ride quality when properly tuned.
On the other hand, take the Bell 214/214ST. A big 2 bladed example. Even with nodal beam suspension to mitigate 2/rev, these beasts ride very harsh and can only be tuned for hover and one nominal cruise airspeed.
Regarding the subject of noise. In the late 80s Los Angeles County Fire Dept, against the better judgement of the pilots, were mandated to change from the 2 bladed, Bell 205A-1 merely for the reason of noise. They subsequently bought 4 bladed, Bell 412s who's performance numbers, on paper, were stated to be equal if not marginally better. The pilots always preferred the performance characteristics of the 2 blades systems. Been on many max gross takeoffs with LACO Fire from Barton heliport, in 2 and 4 blade ships. The pre-takeoff briefing in the 4 blade 412 was not without reference to the possibility of having to pickle the load during translational lift.