Hello and welcome to COL
The general rule of thumb for walking is 5 minutes for each month of the puppies age. So 3 month old Alfie shouldn't be walked for longer than about 15 minutes per walk, although you can do more than one 15 minute walk per day.
The reason for limiting the amount of exercise is to ensure that growing joints and bones aren't damaged. Problems might not be immediately apparent but could show up in later years. Similarly, don't let him go up and down stairs at the moment.
The occasional longer walk is OK but if you're doing that try to avoid hard surfaces like pavements. While Alfie is small you could always carry him for part of the way if you're doing a longer walk.