Thanks for posting. Sorry to hear you're not able to take out a 4th SIM card.
I have 3 SIM's in my name and got told I could have more. I have spoken to someone else who has 4. I've not been made aware of any changes to the amount you can have.
It's possible your credit score isn't good enough to have more than 3 but that's checked on an individual basis so can't say if it's relevant to you or not. I'd imagine you would have been told if that was the case though.
Who told you 3 is the maximum?
Was this done online, at a shop or over the phone?