Do the choices for seats in blue in a plane make difference?

The further you go towards the back of the plane, the less convenient it is on most planes

Because it takes more time to plane and deplane the more at the back you are (the jet bridges are usually in the front and the middle of the aircraft).

It can lead to more passage as well because you are nearer to the aft lavatory

They charge you more for the convenience basically

if each of the 3 has only its middle seats available, which 3's middle seats are better and why?

Well, it depends on what you want, convenience or cheap

For a seat map with reviews, you can look at the seatguru website, which will highlight the worst seats possible

For the first flight: window or aisle is a matter of personal preference, just avoid middle seats.

If you pick an aisle seat, take it in the central section: that reduces the number of times you may have to let your neighbour through by a factor of 4.

Pick a seat in a set if 3 where one is already booked: that will leave only a middle seat, which is usually not favoured, so may remain empty. The further back you are the best chances of this working (but if you are in a hurry take the front most seat you can find).

For the second flight: a middle seat is miserable in any case, but in the central section at least:

• you won’t be bothered by your neighbour in the window seat wanting to get up: both your neighbours have aisle seats and get access the aisle directly;
• You have two different choices of a neighbour to bother to get up.

In terms of row, there are two things to consider:

• if you are in a hurry at your destination it’s better to be closer to the front, as that can save you a few precious minutes;
• if you are towards the end of the meal service you will have less choices, but it can be difficult to guess in what order service is performed. They usually start from the galleys, but sometimes the service pattern is different. You can override this if you order a specific meal in advance.

There are overhead lockers for your carry-on, though they can be quite full in coach (though usually less than in single aisle aircraft). You want to avoid keeping stuff under the seat in front of you: you want that space for your legs and feet! Note also that one some aircraft, some or all seats will have a box under each seat for the IFE (in-flight entertainment, the screen in front of you) which will already reduce your space.

Do the choices for seat in a plane make difference?

They can make a huge difference. There are quite a few different aspects for seat selection but it does vary a lot by airline and specific aircraft (which you didn't provide here). Your best bet is to look up your specific flight on a site like seatguru.com

Things to look for

1. No one wants a middle seat. Some airlines actually charge extra for not-middle.
2. Seat Pitch (distance between rows) and width (distance to neighbor) make a big difference. The more the better but it's very non-linear. Sometimes an inch can make a huge difference.
3. Seat width is mostly a function of how many seat they squeeze into one row. For example, United operates some Boeing-777 with 3-3-3 seating which is quite comfortable. They also have a "high density version" with 3-4-3 seating which is absolutely miserable: you cannot sit straight without rubbing shoulders with your neighbor.
4. Pitch is based on row spacing. Some airlines do have different spacings in economy (and charge for it), others do not. Check seatguru.com
5. Most airlines charge extra for seats in the front. The only benefit here is that you get out quicker at the end of the flight. Sometimes you care, sometimes you don't.
6. Avoid seats close to the bathrooms. It can be smelly and often there is a line of people waiting.
7. Seats close to the galley can also be noisy and a little busy. The quietest area is half way been the galley and the loo.
8. Window vs isle is personal preference. Window gives you a wall to lean against and of course you can look out. However it's more difficult to go to the bathroom.
9. Personally on a long haul with 3-3-3 configuration I would go with an ailse seat in the middle section. Middle seats in the middle sections are the last ones to fill up so this maximizes your chance of an empty seat next to you (which is enormously helpful). And even if it's taken, there is a good chance, it's a couple, so the middle person is less likely to chase you out when they need to go.
10. I personally don't like bulkhead seats since you can't store your stuff under the seat in front of you. You also may have a video monitor in the armrest instead in front of you, so you can't use it during take off and landing.
11. Exit rows seat are a mixed bag. Typically there is more legroom but there can also be limited storage, similar restrictions as bulkheads and limited or no recline. I've also had a few that were quite cold since the door isn't as well insulated as the fuselage. This depends a lot on the specific plane
12. Avoid the row in front of an exit row: these typically have no recline.

UPDATE:

This appears to be Kuwait air 777. https://seatmaps.com/airlines/ku-kuwait-airways/boeing-777/ with a seat width of 18" and a pitch of 32". This is quite good by today's standards.