3 months is way too young for a back pack, wait until they can sit unsupported before using one of these. 6 months approx though this varies.
However baby would be fine in a baby bjorn carrier, probably facing out as long as babe is holding head up by self. You can get a cosy toes attachment for these and they can be worn under waterproofs etc.
After 6 months We had a little life carrier and it was great. did up to 10 mile walks with our 6-18 month daughter with that and when she was toddling she could get out and walk or have a nap in it also
I can vouch for this from both medical knowledge and from personal experience.
However 3 hours seems a long time in a carrier. babies need changing, feeding regularlyetc and they quickly get cold. I’d try a baby bjorn on a shorter walk 1st maybe an hour or so and leave the NCT mentallists to freeze their babies by themselves!