Dot 3 is actually fine too. Hayes did/does use dot 3.
Dot 3 exceeds maguras fluids boiling point by about 100 degrees! But we basically never hear magura owners complain about overheated fluid.
For bike brakes, our pad material typically overheats first and we get greasy brakes. This reduction in performance feels like a firm lever, but no braking power. This means the pads have lost their coefficient of friction to heat, but the fluid is still well under temp. This is enough to stop most riders, its scary as hell to ride with a death grip on the levers and a bike that keeps moving!
Im not saying to intentionally seek out and use the crappier dot 3, but if you need brake fluid, a nice fresh bottle of 5 dollar parts store DOT 4 is way more than sufficient. Valvoline makes an excellent DOT 4 thats available everywhere... but still, you're not going to overheat whatever you put in there.