Fans can draw more power on startup. I would recommend most users stick to 2 fans as well. I have personally used 4 in some rare cases(I do things I do not recommend ) without issues(like yours VERY low power fans).
If the fans are quiet enough for you at full speed. Please use a molex connector to power them to avoid all risk to your boards fan header.
As Sn1992 say, Matching fans is best if they are used for the same cooler(push/pull).
Different fans on the same header are not an issue(it works) for lets say case fans or other fans.
Bejusek, You have just posted on of my favorite adapter styles since you can not overload the motherboard and does not try to split the speed sense/tach cable. The only issue is that 3 pin fans run full speed with that adapter.