Does it burn only when you cum in her or every time?
Cum burns me. Some women's bodies just reject it. It also gives me infections and such.
She could have an infection, but if she has had it for three years I am sure she would know by now.
Try wearing condoms, and using lube and see what happens. Maybe she isn't wet enough and her skin is tearing, or maybe if you do cum in her that is what it is.
If none of that works, then try to talk her into going to the doctor.

Your wife probably has vulvar vestibulitus. I was diagnised with is 4 years ago and am only 24 years old. There are no real symptoms other than a redness and severe pain. She is probably always in pain and is only telling you when the pain is real bad. Because vestibulitus is fairly new to the medical field but also fairly common among women, there is no definite solution for her condition but she is not alone.
Do some research on vulvar vestibulitus. Again...Since there is little factual information as to what causes vestibulitus and how to treat it...Take the internet info with a grain of salt.
Sex is possible. You must wear only lamb skin condoms. They are fairly expensive, but very worth it. Also, tell your wife to:
1. Wear only 100% cotton underwear,
2. Not to use any soap on her vagina. Use only oatmeal soap if she feels it is absolutely necessary. Our bodies have our own fighting power. Rinsing with water is enough.
3. Avoid spicy food
4. Do not wear any underwear when sleeping
5. Use only water based lubricant
6. Go see a specialist who will not tell her that it is a yeast infection. They are wrong!!!!

Good luck

Well, it could be that and it would be worth looking into, but it is not definate.
Soap is a necessity. I would not want to be anywhere near a woman who did not use soap on her cookie. Uggh. Just use natural soap without scents and whatnot, but definately use soap. Please.

Hehe I know this one because it has happened to me too. She probly doesnt get "wet" enough. Lubricant really works!!! I have had the same exact feeling, and lubricant (such as a liquidy kind) extreamly helps!!!!!!!

Please tell your wife to read these posts!!She is not the only woman with this problem!I know she is embarrassed,but having worked in healthcare for 23 years,docs have seen it and heard it all.Maybe she'd be more comfortable seeing a woman gynecologist,more familiar with the territory and all.Please try to help her get over her embarrassment.You've no idea what she's going through,even though it sounds like you are an evolved man .3 years of painful intercourse is 3 years too long and bless her for continuing to make love even when it is painful.Best of luck to the both of you!!Pattyv

Im probably wrong, but it could be cystitus, which is basically a urine infection, does she blled when urinmation sumtimes? If it is then just drink plenty of water too flush everything out dat wot I did.