Is it better to use one adsense unit that gets,let's say 3% CTR or two or more units the CTR of which adds up to 3%?Does the number of ad units make any difference?
The CTR is based on users mindset. Even though you have multiple ads, user has to decide to click or not. It is my opinion that that you have three units in one page as the probability of Click increases.
depends on the size of each ad units, but if you're stuck at 3% then I would rather have a 234x60 ad unit at 3% because this displays only one text ad and so it will always be the best RPM possible, if you're stuck at 3% CTR.
number of ad units does make difference. higher up the ad positioning is, more the clicks will pay. so if you have 3 ads with one being on the top, one in the middle and one at the bottom of the page, the higher position ad will pay highest while the lower ad will pay the lowest. if you can have 3% CTR with only one ad unit, then thats they way you will get highest payout compared to combined CTR from multiple units
from my understanding more impressions with less clicks reduces CPM and CPC ultimately so i would suggest use less ad units and bigger the better