I am multiplexing three different seven segment displays, but they are very dim because of it. How can I solve this issue?
Hi and welcome to the forum.
You can start by showing us your schematic diagram showing all the components and their values, and posting your sketch. Please make sure you read the forum guidelines first so you know how to do those things correctly.
The quick answer is more current (on average). But this must be achieved in a way which does not damage any of the components. It may be that a higher average current can be achieved by making changes to your sketch, if the way the sketch is written causes "dead time" where no segments are lit. We won't know until we see your sketch. It may be possible to use lower value series resistors, but we won't know until we see your schematic.
Two things limit the perceived brightness. How bright it is when there is no multiplexing and what percentage of the time each digit (or segment) is on. If you are multiplexing each segment or if your code spends a lot of time "between" digits with all three off, then they will appear dim.
I already said that. But I think we both wasted our time responding. The OP has not returned.