Deep cleaning the tank and tank decorations most likely only resulted in you removing any beneficial bacteria that was helping your fish. In your post it didn't look like you mentioned doing any water changes. 3 gallons is on the small side for a betta, and adding the plecos is definitely going to make the situation worse. If you do not know about the nitrogen cycle, read this article. You should buy a water test kit as well so you can determine whether the water quality is good or not. Placing it in a tiny bowl like that to "quarantine" it also does way more harm than good. Ammonia, nitrite, and nitrate levels will rise much faster, hurting your betta more. When your betta is the only fish in an already small tank, there is no reason to take it out and quarantine it.