Here are a few ideas.
The good news is that the Corpus 3G seat on the F3 can be adjusted in many ways and is pretty easy to adjust. I recommend getting the Corpus 3G Seat Owners Manual and the Corpus 3G Seat Service Manual. Both are available online for download in PDF form. I will try to attach them to this message. If that does not work, just google for them and you should have no trouble finding them.
I recommend getting a high quality set of hex wrenches with ball ends (I have good luck with Tekton brand). I find some of the folding hex wrenches also work well for some situations. I use a Park Tools brand (from my old biking days) as well as a Husky (Home Depot) brand. Some adjustments require a wrench, and we frequently use a ratchet and sockets for those. An extension bar will come in handy for some hard to reach areas.
The basics, which should have been configured by the folk who provided your wheelchair, are the seat width, seat depth, seat back width, and seat back height. I recommend getting those right first.
For me, It is important that the foot rests be at the proper height. My legs are slightly different in length, so each foot rest is individually adjusted to the length of the appropriate leg. Having this wrong causes me pain in my knee, thigh and hip.
I also use lateral thigh supports. I carefully adjust those to keep my legs from splaying too far out. This prevent soreness in my groin and hip areas.
Having the armrests at the correct width is also very important. Too narrow and my arm falls off the outside. Too wide, and it does not offer enough support. Note that Permobil makes an attachment to keep your arm from falling off the outside of the arm rest. I have that on one wheelchair and like it a lot.
Having the armrests at the correct height and angle are necessary to prevent pain in my shoulders and back. You can adjust the height and angle of the armrests. These can be different for each arm rest.
I need a narrow seat back (16 inches). In addition, I really need lateral thoracic supports. Those provide a lot of stability to my trunk and are a great benefit. They are hard to adjust just right. For me, I have them adjusted so they engage the lower part of my rib cage.
One of the most critical and hardest to adjust items for me is the head rest. I have used several different ones. I find the newer style of the F3 and F5 to be pretty good. I like my headrest pretty far forward so that I can always have my head supported.
I pay very close attention to the position and orientation of the joystick. I have mine set up so that my hand naturally falls where the joystick sticks up between my thumb and index finger. I have more strength and dexterity in my wrist than in my fingers, so it works better for me to have the joystick near the base of the thumb and index finger.
I really like the Roho Quadtro Deep seat cushion. I can sit on a foam cushion comfortably for about 30 minutes. After that, I really need the Roho. When I started out, I could go all day on the foam cushion, but those days are over.
I also find that the "seatbelt" (actually, a postural support device) is important for me. Without it, I creep forward in the wheelchair and eventually my thighs really start to hurt. I prefer the unpadded seatbelt, as it is more supple and easier for me to handle. I really like the Permobil seat belt that has the loop on the end that I can stick my thumb through to adjust the tightness. I have my seatbelt set up so the buckle is on the right half. The length is adjusted (at the side rail) so that the I can put my thumb through that loop and tighten the seatbelt with a simple pull of my left (stronger) arm across my body. It takes very little strength to do that.
I prefer the ride of pneumatic tires, though if you go that route you need to be prepared to deal with a flat. The foam filled tires have a harsher ride and are heavier, but with them you will not run a risk of getting a flat. I often run mud and snow tires (or on the F5, the Permobil aggressive tread tires). We get lots of snow here!
I encourage you to set up the wheelchair fit while you still have some mobility. Doing so requires getting out of the chair for many of the adjustments. That is much easier to do when you still have some mobility.
Be sure to post questions as you go through making the adjustments. I will be glad to help as I can, as I am sure many others will.
Steve