Deworming every 3 months may be sufficient or it may be woefully inadequate depending on your horse and his particular situation. It's really important that we stop just guessing at deworming and treating every horse and situation like it's the same because in doing so we increase the risk of heavy parasitism in our horses, heavy pasture contamination and faster developement of drug resistance. We need to assess each individual as an individual--some are in situations where the risk of reinfection is very low, some are in situations where the risk of reinfection is very high, some have a high natural resistance to parasites while others have very low natural resistance to parasites. So what is good for one horse may not be appropriate for another horse even in the same pasture.
With deworming every 3 months and rotating dewormers in an area where the weather conditions are right for parasites to survive, you have a 2 month window when there can be hundreds of thousands of parasite eggs shed each day into your pasture. If you are in a situation where your horse is stalled all the time and fed hay/grain and gets turned out in a small paddock that is picked daily, you may still be over-deworming. You may only need to deworm twice a year. And since you aren't doing fecal egg counts occasionally, you may be using drugs that aren't effective in your area--for example, there is very widespread resistance to fenbendazole among strongyles. So if you deworm with fenbendazole and there are resistant strongyles, you can have egg shedding for 12-14 weeks---that's alot of eggs!
Licensed Veterinary Technician