Question

What is series combination?

Answer 1

When all the devices in a circuit are connected by parallel connections, then the circuit is referred to as a parallel circuit. A third type of circuit involves the dual use of series and parallel connections in a circuit; such circuits are referred to as compound circuits or combination

Answer 2

In a series combination the electric components are connected end to end along a same line. It is such that the value of current in a series circuit remains same. There is no branching of current.
Batteries are connected in series to provide large value of voltage.
The net resistance in a series circuit is the sum of the individual resistance and is thus greater compared to parallel combination.

Now, if R1, R2 and R3 are connected in series, as shown below: (Check this attachment)

the effective resistance will be

R = R1 + R2 + R3