Being that you seem to be a novice at low-intensity exercise. Walking is a great step in the right direction. Exercise as simple as walking can improve cardiovascular health, increase mobility, and at 3 miles should have a positive metabolic response.
Usually walking any distance is one of the easiest things a person can do but there are several factors that could bare some limitations on your little adventure.
Are you flat-footed? Do you wear shoes with arch supports? Do you not have arch supports? Have you had any injury to your legs or feet that would involve overcompensation in your lower leg muscles?
If you are carrying over 100 lbs of additional body weight this could be cumbersome on muscles when brought into practice. Calve muscles should naturally be somewhat strong unless you lead a sedentary lifestyle.
What kind of terrain are you walking on? streets, sidewalks? trails, mud puddles? Where are you walking?
If you can be honest with yourself about the above issues, and do a self-analysis on how, where, and why you're walking 3 miles a day. Then you can proceed to develop those calf muscles as mentioned above.
Eliminate the pain
In order to eliminate some of the pain you are currently experiencing, you could soak in Epsom salts, half cup salt/2 quarts water is my personal recipe, works wonders on sore joints and muscles.
Do some stretching of those hamstrings and calves by sitting down on a yoga mat or floor, with feet straight out in front of you, no bends at the knee, put your arms above your head, also straight up, and bring them slowly down to try and touch your feet, hold on to your shins if you can't reach toes, flex toes backwards towards your fingers to stretch the calf. You can also do: