"'Be strong and very courageous. Be careful to obey all the law my servant Moses gave you; do not turn from it to the right or to the left, that you may be successful wherever you go. Do not let this Book of the Law depart from your mouth; meditate on it day and night, so that you may be careful to do everything written in it. Then you will be prosperous and successful. Have I not commanded you? Be strong and courageous. Do not be terrified; do not be discouraged, for the Lord your God will be with you wherever you go.'" NIV translation
The Lord tells him to be strong and courageous twice more and assures him that, if he follows the ways prescribed by God (the Law), he will be successful. As the soldier was getting ready to go into battle, God was reminding him that He would always be there with him.
This is very similar to what Jesus told the disciples in the Great Commission where He said that He would be with them (and us) until the very end of the age. Our Christian walk (promised land) is a fight as it seems that there are enemies of the gospel on every side but, like Joshua, we can be assured that He is in the battle with us.
"So Joshua ordered the officers of the people: 'Go through the camp and tell the people, "Get your supplies ready. Three days from now you will cross the Jordan here to go in and take possession of the land the Lord your God is giving you for your own."'" NIV translation
Why did they wait three days to enter the land? They were to get their things ready but this was also a time to count the cost of listening to the Lord. Grabbing the gift was not going to be easy as there were many battles to come. Jesus described the same thing when He was teaching the people about the cost of being a disciple. (see Luke 14:28)
When we come to Christ for salvation, we must also understand that we are going to face many battles as well. There will be internal battles with our own flesh and desires but there will also be external opposition from those who do not know the Lord.
"But to the Reubenites, the Gadites and the half-tribe of Manasseh, Joshua said, 'Remember the command that Moses the servant of the Lord gave you: "The Lord your God is giving you rest and has granted you this land. Your wives, your children and your livestock may stay in the land that Moses gave you east of the Jordan, but all your fighting men, fully armed, must cross over ahead of your brothers. You are to help your brothers until the Lord gives them rest, as he has done for you, and until they too have taken possession of the land that the Lord your God is giving them. After that, you may go back and occupy your own land, which Moses the servant of the Lord gave you east of the Jordan toward the sunrise."'" NIV translation
These two and a half tribes had been given land on the east side of the Jordan River but were to go ahead of the rest of Israel into the battle for the promised land. Throughout the Bible, the direction of east is associated with the idea of being away from the will of God.
This reminds us of the fact that we are to tell people of the way of salvation. Then, we are to build them up in the faith so that they can grab onto the life that God has promised them.