Can you add some DOT3 to DOT5 (or vice versa) to temporarily top off the MC?

Y E S ! ! ! !

Will they freely comingle their molecules? NO. They'll always separate out in to two fluids. There is no benefit from mixing fluids of different DOT ratings, but the Federal regulations that govern brake fluids REQUIRE that they can be added together and not affect the basic function of the brake system.

If you need to top off the MC to get the car home, do it. Then replace all the fluid with what you need ASAP.

Clair