The phrase used to (do something) suggests a continuity of practice (from 'use'), a habit, a matter of course, a regular feature, or something customary in nature compared to have (something) that has a static/ momentary implication.

I used to have three cats

Having three cats was a natural, it was always so, for some time, was nothing unusual at that time, I was accustomed to having three cats, …

I had three cats

That's it. I had them. At a point of time. I am not saying anything more about it. May be it was for a day, may be for an hour, or for a year.

See [TFD idioms][1]:

A young lady who used to work in my office had seven brothers!
We used to visit our parents at Christmas every year.

Compare: used to work (a routine) & had seven brothers above.
Notice: used to visit in reference to at Christmas every year. (customary)
[1]: http://idioms.thefreedictionary.com/used+to

The phrase used to (do something) suggests a continuity of practice (from 'use'), a habit, a matter of course, a regular feature, or something customary in nature compared to have (something) that has a static/ momentary implication.

I used to have three cats

Having three cats was a natural, it was always so, for some time, was nothing unusual at that time, I was accustomed to having three cats, …

I had three cats

That's it. I had them. At a point of time. I am not saying anything more about it. May be it was for a day, may be for an hour, or for a year.

See TFD idioms:

A young lady who used to work in my office had seven brothers!
We used to visit our parents at Christmas every year.

Compare: used to work (a routine) & had seven brothers above.
Notice: used to visit in reference to at Christmas every year. (customary)