I have (2) different 3.5 inch squares among my many rulers and I use them a lot.
One has a lot more lines, it is meant for people wanting to do miniatures and precise stuff and is too busy for me to read easily, but if all I'm using is the 3.5" dimension that doesn't matter so much. The other one has less lines and I like it better in some ways, but again, if all I care about is the outside dimension they don't matter too much.
One of my favorite "tools" is a roll of 1/4" masking tape, it can be used for many quilting related things, I often mark the cutting line of my ruler with piece of tape to help me both with my vision as well as my consistency issues.
There are also cutting systems that have dies. Some people love them, I have friends that do but I haven't felt the need myself.
We also have sheets of plastic grid that you can use to make templates, I do a lot with graph paper and the plastic, but you need to know in advance it is not meant for cutting around. You can draw around it easily, you can butt a ruler up to it and use a ruler, but that is really too thin to safely or repeat cut multiple pieces.