Electric circuit is built in specific way because resistors create letter H. They look like H brigde. Designation of total resistance of this circuit is quite difficult. We can imagine that its side branches create voltage dividers. If that voltage dividers are equal no current will flow through resistor R3. In that specific situation we can omit resistor R3 in total resistance calculation and treat circuit like there is a break instead resistor R3. You can download simulation in pspice and calculation in Excel to examine circuit behaviour.

simulation in pspice and calculations in Excel