What impression do you form about the three friends regarding their friendship with ach other

I think all three men are very comfortable with each other. They accept each other's foibles and  eccentricities. They certainly point them out and tease each other but there is an understanding that they are close friends above all and appreciate each other's company. 

On the face of it, the three friends seem to get quite easily irritated with each other. They are often shown sparring and blaming each other for things that go wrong during the trip (and before).

This is neatly encapsulated in an exchange in chapter 15, when the three all accuse each other of failing to do their fair share of work on the boat. Harris first complains that George and Jerome have been ‘imposing upon him’, and George retaliates by labelling both Harris and Jerome as ‘a pair of lazy skulks’. Harris retorts that he’s never seen George working at all, and appeals to Jerome to support him in this. Jerome does so. George then accuses Harris of sleeping all the time and gets Jerome to agree with him. Finally, both George and Harris turn on Jerome, labelling him as the laziest one of them all.

Jerome reflects:

And that was their gratitude to me for having brought them and their wretched old boat all the way up from Kingston, and for having superintended and managed everything for them, and taken care of them, and slaved for them. It is the way of the world. (chapter 15)

Jerome here sounds quite hurt at the others’ lack of ‘gratitude’ to him, but in fact he is being ironic; as so often in this book, he is really having a laugh at himself and at the others. He knows that none of them have worked particularly hard, and certainly not to the extent that he claims for himself here. He is, essentially, remarking on the human tendency to blame others for problems and ills.

The three friends, then, often quarrel in this story, but it is never serious, and their interactions make for a lot of the comedy in the book. In spite of their frequent arguing, we get the impression that they are good friends, who clearly have known each other for a long time. Although things may go wrong on the trip, they share a bond of close companionship throughout.