Hi!
There are a few issues to discuss here.
First off, while you don't have tether anchors in the rear, you do have one in the front seat! It's a hole in the side of the leg and can be used to tether a forward facing seat installed in the front seat. You may also have a keyed on/off switch for the front airbag. Even if you don't, I would choose tether in the front over no tether in the back once rear facing is outgrown if I were you, as long as that doesn't compromise safety for others.
Second, you do have weld nuts in the back wall marking points for installing tether anchors; while Ford doesn't have parts it may be possible to use one supplied by a car seat manufacturer.
Third, I'm most concerned about the size of your rear seat. I looked it up and it's extremely small; I'm unsure of which seats will be able to install properly with 4/5 of the seat base on the vehicle seat, as required, especially with a seat to rear face as long as possible. Your best bet is probably to start with compact seats and try them out one by one until you find one that fits. The Graco seats may be a place to start, as they are usually fairly compact, but I'm honestly not sure which will be best. You may end up needing to install in the front seat, if you do have that keyed off switch for the airbag.
Fourth, you mention a 3-in-1 seat. I think you would be best served by looking for a convertible and not worrying about booster mode; not only is this usually cheaper, the high back booster mode on these seats is often short lived and/or difficult to buckle, and this vehicle is surely no exception. Also, I can't think of one that will be 100% oh the vehicle seat- required in booster mode. Backless boosters likely won't ever be appropriate in the rear seat of this truck because of the lack of head restraints in the vehicle; children need support to the top of the ears either from a booster or the vehicle to prevent whiplash. So focus on the car seat part, and then deal with the booster issue later.
Lastly, you mention siblings- who else needs to ride in this vehicle regularly?