By increasing p.d. across the ends of a conductor, current flow can be increased. But the increase in the amount of current flow depends on the conducting ability of the conductor. Some conductors offer some resistance to current flow than others.

Resistance (R) of a conductor is defined as the ratio of potential difference (V), across the conductor to the current (I), flowing through it.

- SI unit of resistance is the ohm $\Omega$.
- V = IR

Ohm’s law states that, the current flowing in a metallic conductor is directly proportional to the potential difference applied across its ends, provided that all other physical conditions, such as temperature, are constant. Comparing with V=IR, thus, R must be constant for a metallic conductor under steady physical conditions.

i would like to read online and i have a very serious issue on calculating resistance when more than three resistors are in parallel

For the calculation of effective resistance for three resistors in parallel, you can read up on Effective resistance of resistors.