Oregano can be divided anytime, but preferably spring as new growth emerges or fall as you are preparing your beds for the winter. Take a sharp spade and first dig out one half of the clump, then cut the remaining clump in half again...or not. The idea is to remove the old growth.

Chives are best divided in spring, also as new growth emerges. One way to thin them out and rejuvenate at the same time is after they bloom and before the flowers go to seed pull out the flower stems from the crown removing the entire stem and roots. Then dig the clumps you want to remove.

If lavender is to be rejuvenated it is best done early spring just as new growth emerges. But you can start the process as soon as it finishes blooming by first trimming back the flowers, then prune only old or dead growth by pruning out no more than 1/4 of the plants current size, shaping it into a mound. I've read where you can prune it back again in November but I prefer to wait until early spring. You may want to verify the mature size of the variety you have because some lavenders, like grosso, are large plants.

After you have divided the oregano and chives you can pot up your excess and sell or donate the plants to any organization that may take them.