Guinea pigs are not natural climbers like rats, hamsters - they are ground dwellers. Their spines are not really adapted to climbing.
Some WILL learn to use a ramp, others will never do so - I've got some hutches with ramps here and I have to say they are my least favourite, the ramp as Bonnie mentioned takes up floor space on both levels.
With C&C I guess you need to make the main living area and adequate size for the pigs (you're getting 3 aren't you? So 5' x 2' at least) and if you make a hay loft and then use it then it's a bonus.
There are all sorts of ways of making the ramps safer or easier to use, such as lining them with rubber matting, carpet or a fleecy tunnel, Ryli on here has got a great tunnel down her ramp and they use it now - take a look in the cage and hutch gallery.
As long as you're aware they might not use it, there's no harm in trying as long as the main part of the cage is big enough if they choose not to
Sophie
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