What is the effect on strength of concrete if an RMC transit mixer is halted for 3 hours?

The first thing we need to know is the amount of retarder used for the preparation of concrete. Generally, the concrete starts to harden after 30mins of adding water into it.
By the use of a retarder, the setting time of the concrete is delayed. Generally, for RMC mix concrete the setting time starts after 4hrs.
Thus, in your case, there shall be no effect on the strength of concrete.
But it is advised to check the slump of the concrete before pouring.