Unfortunately Half-Life doesn't provide something like a trigger_random entity, but eventually some mapper(s) came up with a technique to trigger random entities. By using a high-damage
env_beam that randomly targets one of several
info_targets, and by placing
func_buttons in front of those targets, you can trigger different entities depending on which info_target the beam decides to strike. It's described in more detail in the following tutorials:
A Random Event Generator and
Random entity triggering.
If you want to trigger multiple things at the same time, you can give them the same name (so they'll always be triggered together), or you can trigger a
multi_manager, an entity that can trigger multiple entities (see
Tutorial: Multi_manager for more info).