look into which one you get though as they vary a great deal.
Some keep you warm some cold. Also think seriously about wearing something for 3 days close to your skin. Your body gets rid of the toxins for a reason. Just because it doesn’t smell doesn’t mean its good for you.
A combination base layer that is both synthetic and merino will give you the better performance than a 100% merino as you get the best of both worlds. merino keeps you comfortable and the synthetic fibres (generally polyamide) dry it out.
9 times out of 10 a polyester base layer is better for high intensity activity as it doesn’t hold moisture like merino does.
Personally for a 3 day ride I’d look at a couple of light weight polyester base layers, They are super light weight and take up a lot less room than a merino one, Plus you get to wear a fresh top each day.