Its hard work because it is a back killer, a knee killer and some horses really set to leaning on the farrier. In addition, if you have a horse that pulls its feet away or is "light on the feet" it can result in torn muscles or bruises.
I think its a bit odd that you have three farriers who don't call you back. I have had two farriers in the last 15 years. One up and stopped calling me back. The second one calls back the day I call him (had him about four years now). We did spend some time doing our own horses feet. We have 8 and between busy schedules and bad backs they went too long. The fact was that after an 8+ hour day at work, I want to ride not trim feet.
I would look at your horses, your distance and your behavior with your farrier. Are your horses well behaved? Do you have them ready for the farrier when he arrives or does he have to help you catch them? Do you pick out their feet prior to him trimming them? Do you pay him before he leaves the farm? I ask these because it seems to really make a difference for the farrier. Upon arrival the farrier appreciates that our horses are caught, feet are clean, legs are cleaned and they all stand nicely for him. We had a mare who was/is a bit of twit to catch and was really bad about having her feet done. For the first few trims, I had her tranquilized when he arrived. After that I made it my job to improve her behavior, she now stands well for him untranquilized. Most importantly pay the farrier before they leave the farm. Our farrier has vented to us (we are all in the horse service industry) about owners who demand he rush out there for a pre-show trim and than ask to mail the cheque because they don't have money due to the show. Don't send a cheque give it to him. It makes a big difference.