Theoretically, the hose trick should find any suction-side leaks above ground, including the valves or the o-rings. Did you hold in each area for a good long while?
You're already on to the next step. Changing out the o-rings is easy and cheap, so give that a go. When I fixed my leaking valve, the o-rings came in a five-set pack. See if you can find the o-rings for your valves, along with a video about how to change them out. I found fixing mine to be pretty easy. Then the same for the pump. There's the big ring for the lid, but there may also be an o-ring on a plug near the bottom.
Or you can try this first: after you take a pic of where the valve handles are normally, try the various combinations that would isolate each of the three branches of your suction side plumbing. If plumbed correctly, there should be no way to set the valves such that you would dead-head the pump (cut off all its water sources), but just in case, move the valves slowly and listen for any drastic change in the sound of the pump, as if it was being choked off. You're looking to see if by cutting off all the pipes, one at a time, if the bubbles stop. That would indicate which of the lines is leaking. If that works, you can try the hose trick again on the length of pipe between the valve and the ground. If that doesn't reveal the leak, then that kind'a indicates the leak is underground, unfortunately. Though there is a "glue trick" or a plastic wrap trick we can try if this test reveals the leak is in only one line.
The plastic wrap trick is basically wrapping PVC joints with plastic wrap to see if that stops the leak. You can do one joint at a time, or all of them to rule out multiple leaking glue joints. The "glue trick," which I don't personally like, is squirting PVC glue at the suspected leak in hopes the system's suction will draw into the leak enough glue to plug itself. I'd prefer to replace the joint if possible, but others seem to think squirting glue is an adequate fix.
You are also correctly theorizing that there could be more than one leak. So you might have to try some or all of these tricks in various order.