It is okay to add DOT3 brake fluid.
The brake light came on again because of your low brake fluid. If your brake fluid is getting low as soon as you said, you might have the leak somewhere.
Try to top it up and drive to a good mechanic place and have them checked, be sure you have good brake pressure or if you need to stop, you might wreck your car........
PS: since you see something from the driver's side rim, make sure if they're brake fluid and tell the mechanic about it.