M.G.
Sharing anchors or top tethers is NOT allowed/safe. If your middle seat does not have it's own set of lower anchors and top tether you must use the seat belt not LATCH. You can move your eldest child outboard to behind the passenger and use the lower anchors and top tether provided for her seat. You can test out the seats placed where you want them but make certain that each of them is correctly installed and not using either for support. It is perfectly safe to use either a seat belt or LATCH for install as neither is safer if installed correctly. Please do not place two seats onto the same anchor or tether as that could prove quite dangerous. The middle seat is the safest as it is furthest away from impact however the safest place for any child in the car is in a seat that is properly installed.