Why do you only want a phone that can use 3G?
If you do not live in a 4G coverage area, a 4G ready phone would automatically down-shift to 3G anyway.
Many new phones are 4G ready, although some of the Alcatel models are not 4g, 3g only but the quality on them would not be that great. You could look to pick up an old iphone 5, but that too, would be very old now and not receive any updates.
I personally would buy the phone I like, one that suits my budget and then just use it. You would be able to tell the phone in it's settings that you only want it to connect to 3G (not 4G) anyway. This can be done from phone settings>mobile connection>preferred network mode (or similar, depending on your phone) see my screen shot below from my Honor 10. I would recommend Honor phones for a Flagship level phone at a fraction of the cost - and if you want, you can lock it to 3G too..
Hope this helps