You can swap chainrings, but without knowing the BCD (Bolt Circle Diameter) of your chainrings, we can't possibly tell if a 36 would fit in your 39 ring position. It's likely that you have a 130/74 BCD road crank, so 39 is the smallest middle ring you can manage. I don't see much utility in replacing the two outer rings unless they're just plain worn out- you won't get any additional gear ratios, and you'll lose 2% on the top end.
You can get a ring as small as 26 for your inner ring- I've done this. The 26 will feel 12% easier on hills- which is about an extra gear. Sometimes this is enough. If it isn't , then you can get a mountain bike derailleur and cassette, which will give you lower gearing if necessary.
The rear derailleur problems could be from a number of issues, all of which need to be straightened out if you want clean, crisp shifting:
1) Most likely it's a sticky cable. Dissassemble, clean, lube, adjust cable and stops.
2) Bad cable tension adjustment. Adjust cable and stops.
3) Bent or loose derailleur hanger. Inspect, replace or straighten, adjust cable and stops.
4) Least likely, your derailleur is just worn out. Repalce, adjust cable and stops.