Pool Closing Issue

[amped16]

New pool owner here!! In Maryland... I am attempting to close my in-ground pool in my newly purchased house. I do not have a multi-valve (just the skimmer valve into the basket, which leads into the DE filter, then there is a return valve back into the return lines). I backwashed the DE filter. Then opened the filter and cleaned the DE filters, re-closed the filter housing and put the clamp back on. Opened back up the return line, then used a shop vac connected in the skimmer to blow out the return lines (there are 3). I was told (by the previous owners) I do not need to do anything with the main drain - it does not have a separate valve. So once I plugged the return valves, unplugged the shop vac from the skimmer, the skimmer starts filling up with water (about 3 inches). I do not think the skimmer should be filling up with water and feel like the return valve should be shut off at some point? Something is not right and hoping someone can give me some advice here! Thanks in advance