‘ And to the Reubenites, and to the Gadites, and to the half tribe of Manasseh, Joshua spoke saying.'
These were the tribes who had requested permission to stay in Transjordan and settle there. Moses had agreed, after much heartsearching lest it displease God, on condition that they assisted in the capture of the land (Numbers 32:1-27).
So this was a solemn formal approach by Joshua as he gathered the leaders of the three tribes together to establish their commitment to their promise in the form of a covenant. It was a formal swearing of loyalty and obedience to Joshua in the task that lay ahead, binding them in the sight of YHWH on penalty of death.