All seats can be installed with the seat belt, so no worries there! If your younger grandchild is still in an infant seat with a base, she probably won’t be for long, so I would recommend buying a convertible seat. That way you won’t be buying a base that you won’t be able to use for very long.
The three year old could be a bit harder. Since your truck doesn’t have LATCH, the lower anchors you looked for and top tethers, it would be much safer if this child could be rear-facing as well. This is because the top tether attachment keeps the top of the seat from moving forward as much in a crash, which reduces the chance of head injury.
If you can tell us how tall and heavy each child is and how much you can spend on seats for them, we can give more specific recommendations.
Edit: I was just looking at photos of the truck you have, and it looks like you may have top tethers. If you have your manual, read the child seat section for more information. Most manuals are also available online. If you do have top tethers, that would make forward facing much safer!