If you're mapping for CS, forget everything you know about Source entities. Unfortunately, there are severe limitations. Goldsrc doesn't have the input/output system of Source and not many entities reset from round to round. It's just something you have to work around.
func_door is your friend.
func_door resets every round. So, if you want to trigger something on round start, use a hidden func_door. It can just be a little null textured cube in an unreachable room somewhere on the map. All you have to do is set the target of the func_door to whatever entity you want triggered on round start. Use a multi_manager if you need multiple things to happen. I have a complex system of multi_managers and trigger_changetarget entities to create this round setup on my latest map. https://twhl.info/vault/view/6279
(note that the func_door in this map also needed a name so that each trigger_changetarget could change it's target to the next rounds multimanager). When using multi_manager, NEVER use the multithreaded option. Your map will crash in no-time if you do this. If what you are trying to do requires it to be multithreaded then what you are trying to do is a bad idea in a CS map (it doesn't necessarily mean your intended result is a bad idea, it just means, in Goldsrc CS, it's not happening).
I can't remember every entity that resets off the top of my head so you'll have to play with them. Most of them don't have a target so func_door is the most reliable round start trigger. func_breakable always resets, so whatever gets broken will come back each round. func_train is really quirky in cs, it doesn't seem to fully reset, it keeps doing it's thing but jumps back to it's first stop target at the start of each round (I used a constantly running func_train to periodically trigger a bird sound near CT spawn in my map de_hollis, but because of this weird quirk, and because I have the sound triggered on the first stop target, the sound always plays at the start of the round).
trigger_once only works once per map, and never again. trigger_auto fires on map load and that's it. I've never had to do this, but you could tie a trigger multiple to a game_counter, set the initial value to 0, set the limit value to 1 (this means it will fire it's target when it hits 1). Afterwards, the trigger multiple will keep triggering the counter but it already hit it's limit and won't fire again. Then have a hidden func_door trigger a game_counter_set that resets the counter to 0 on round start. That way, you could have a trigger that can only fire once per round. I've never had to do this, so hopefully there aren't any unknown quirks that will get in your way, but it's worth a shot.
The setup would be as follows:
Put your trigger_multiple that you only want to fire once per round wherever you want it to be. You can use whatever names you want, these are example names. Set it's target as "oneoff" and delay before reset to 1. Create a game_counter, set it's name to "oneoff" and set it's target to whatever you want to target (I'll just call it, "thing" for this example). Set the initial value to 0 and set the limit value to 1. Create a game_counter_set, set it's name to "oneoffset", set it's target to "oneoff", and set the new value to 0. Then finally, create a hidden func_door (can just be a random cube hidden somewhere) and set it's target to "oneoffset".
Edit: I just remembered another big one, light entities reset to their initial state on each round. You can't target anything with them but it's important to know if you're making any setups using switchable lights.