Question about this RC circuit and voltage across the capacitor
After it's fully charged or discharged (i.e. at DC), a capacitor acts like an open circuit. That means there's no current through R3, which means there's no voltage across R3. That's just Ohm's Law. Since \$V_{R3} = 0\$, \$V_C(0^-) = V = 12 V\$.