The second flush of blooms that comes on the new wood isn’t usually as full as the first, but it’s still great and gives you color in the garden.
The key to pruning these is to do it lightly as they don’t need much. Lightly prune branches and dead flowers after the first flush of growth in the spring and then again lightly prune after the second flush of growth.
The only reason I might prune group 2 is that if they become straggly or out of control.
Varieties of Clematis in group 3 bloom in the latter part of the season on new wood that is formed during the current growing season. These typically are considered the fall-blooming Clematis and flowers show up in late August and early September.
These can be pruned in the very early spring or even late this fall after blooming. They flower on new growth, so you can cut them back pretty far and still have a nice plant.
If they are overgrown, they can be cut back by more than half and still put on a spectacular display.
The other question that is related to pruning Clematis has to do with pruning Hydrangeas. Here again, it depends on what variety of Hydrangea you have.
Some bloom on old wood that developed last year (oakleaf) and some bloom on new wood that grows this season (Limelight, Vanilla Strawberry).
The rule of thumb is that if a plant blooms on old wood, prune after blooming; if it blooms on new wood, you can prune in the early spring before new growth begins.