This is repeated 3 times (once for each soldier):

1. two girls cross

2. one girl brings boat back

3. soldier crosses

4. second girl brings boat back

If the girls don’t stay with the soldiers, it only takes 9 trips; the first 8 trips to get the first two soldiers over, then just one more to get the last soldier across.

2. One soldier takes the boat across the river. The other girl takes the boat back.

3. Repeat for each soldier.

So, it takes 2 trips over and back for each soldier. You don’t mention where the girls end up. If they remain on the other side of the river from the soldiers, where they started, it takes 3 trips over the river and 3 trips back. If the girls want to end up on the same side of the river as the soldiers, there will be 1 more trip over the river. If one girl remains with the soldiers and the other on the other side of the river, it’s only 3 trips over and 2 back.

The girls get out of the boat. Soldier A gets in, ties the rope to the boat and rows to the other side of the river and gets out.

The girls pull the boat back across the river so Soldier B can get in and repeat the process. They repeat it again for Soldier C.

Finally, the two girls get back in the boat, row across the river and give the soldiers their rope back.

Next a soldier brings the boat across. He gets off and the girl on the other side gets in and brings the boat back.

The two girls get in the boat again – and drop one girl on the other side. The other girl goes back to where the two soldiers are. She gets off.

Again, a soldier brings the boat across. He gets off and the girl on the other side gets in and brings the boat back.

Next a soldier brings the boat across. He gets off on the other side and the girl who is there gets in and brings the boat back.

The two girls get in the boat again – and drop one girl on the other side. The other girl goes back to where the last soldier is. She gets off. The soldier brings the boat across. Now all three soldiers are across.

The girl with the soldiers gets in the boat and goes back to the other side where her friend is.

First the two girls go across. One goes back and sends a soldier across. The girl on the other side helps the soldier out of the boat and goes back to where the two soldiers and other girl are. Both girls cross again. One girl returns and sends a second soldier across. the other girl helps him out and returns across the river. Both girls cross again. One returns and the last solider crosses. The other girl helps the soldier out of the boat adn crosses the river. Both girls cross adn meet the three soldiers on the other side.