Hi, welcome to the forum 
Wow, your definitely right, the sizing between the 2nd and 3rd plug is very large and id personally recommend buying a plug with a circumference between the two as you may find your struggle with such a large jump in size. Something like 4.5” curcumference would be a good bet.
Sizing up can take a while, so you dont need to rush things. Just take it at your own pace and get comfy with the plug your using before trying a larger size.
Also, i know youve probably read this ad nauseam, but buy a good quality anal lube thats compatible with your toys and dont be afraid to use it. Lube is really important for anal and makes everything much more comfortable.
P-spot toys have a curved shape, so no theyre not going to hit it im afraid. Theres an excellent thread on p-spotting with lots of recommendations and tips Here
Another option for p-spotting is using a curved dildo. The njoy pure wand is the iconic one, but there are other, less expensive, options out there too (This one springs to mind)