How do you insert 3.5 inch disk into apple computer that appears to only accept 5 inch?

The answer is probably "You don't". Macs have not supported 3.5 inch floppy discs for many years. If you need to use such a disc with a modern Mac then you will need to buy an external USB floppy disc drive. If you mean a 3 inch mini DVD then again you will need some form of external drive to use such a disc or an old Mac with a pop out drawer to hold the disc. If it is a disc from a camera then you can usually connect the camera to the Mac and read the disc from the camera. Full details of the type of disc and Mac would help.