If its an older model car then the necessary third anchor point probably isn't there. If it isn't then I'm afraid there is no way to safely or legally fit a 3 point belt. Please don't be tempted to get a "helpful" garage to just make one fit by welding a third point on, it won't do its job if you have an accident. I'm sure you wouldn't even consider this especially as its for child seat.
Transporting children around can be a major hidden expense of having them. I've only recently had to buy a bigger car due to the impending arrival of a new addition. Didn't want to do it as the previous one was old but working perfectly, just too small for 2 children, buggy, travel cots etc. With 3 I'm sure its even worse!