I am a doctor and a 25-year-old who masturbates three times a day falls well within the normal range. Anyone who says otherwise has a non-medical viewpoint.
Short of low testosterone levels (and your doctor can test for this), frequent masturbating in itself should not decrease erection strength. Most men do not see a substantial reduction in erection intensity until their 40s.
You mention your erection is not as strong as you feel it should be - is this when masturbating or during sex?
All men have different ways of masturbating. Some use a very firm grip and use a very quick stroking tempo. In sex, if the vaginal/oral/anal action of your partner does not equal what you've trained your penis to enjoy, you may find your penis is not stimulated enough.
If you are experiencing this only in masturbation, there could be a couple of factors associated with a weaker erection. Your penis may have become "bored" with the same old thing.
In either situation, consider using a lighter grip and a slower stroke. You might try different forms of lubricant. Your masturbation sessions will last longer and you may find that your erections will return to the level you desire.
The other thing you may want to consider is limiting your use of pornography in masturbation. The use of porn may cause you to look for newer and newer media with which to stimulate your mind. Use your imagination - it offers infinite variety.
Good luck.