how can lumber destroy the pleasures of camping in chapter-3?

In this context, lumber means overpacking useless items that become sumbersome during what is supposed to be a trip of leisure. These things make the boat to heavy to pull and tire the rower.

George comes out really quite sensible at times.  You’d be surprised.  I call that downright wisdom, not merely as regards the present case, but with reference to our trip up the river of life, generally.  How many people, on that voyage, load up the boat till it is ever in danger of swamping with a store of foolish things which they think essential to the pleasure and comfort of the trip, but which are really only useless lumber.

How they pile the poor little craft mast-high with fine clothes and big houses; with useless servants, and a host of swell friends that do not care twopence for them, and that they do not care three ha’pence for; with expensive entertainments that nobody enjoys, with formalities and fashions, with pretence and ostentation, and with—oh, heaviest, maddest lumber of all!—the dread of what will my neighbour think, with luxuries that only cloy, with pleasures that bore, with empty show that, like the criminal’s iron crown of yore, makes to bleed and swoon the aching head that wears it!

It is lumber, man—all lumber!  Throw it overboard.  It makes the boat so heavy to pull, you nearly faint at the oars.  It makes it so cumbersome and dangerous to manage, you never know a moment’s freedom from anxiety and care, never gain a moment’s rest for dreamy laziness—no time to watch the windy shadows skimming lightly o’er the shallows, or the glittering sunbeams flitting in and out among the ripples, or the great trees by the margin looking down at their own image, or the woods all green and golden, or the lilies white and yellow, or the sombre-waving rushes, or the sedges, or the orchis, or the blue forget-me-nots.

Throw the lumber over, man!  Let your boat of life be light, packed with only what you need—a homely home and simple pleasures, one or two friends, worth the name, someone to love and someone to love you, a cat, a dog, and a pipe or two, enough to eat and enough to wear, and a little more than enough to drink; for thirst is a dangerous thing.