Oh Martham, don't beat yourself up over this. I mix-fed ds1 and felt guilty as hell about it - and he has turned out a healthy, extremely intelligent, well-grown child.
If you would feel better going over completely to formula (and, from my experience having mix-fed for 5m, it can be a relief to hand over responsibility to the bottle) then that's fine. As others have said, you have done brilliantly to bf to 3m. OTOH, if you want to continue with the bfing, do please go ahead at whatever rate feels right. Even bfing at every feed before supplementing gives LO bm, plus cuddles, cosiness and comfort to both of you, as well as fulfil the baby's desire to suck. Try in this case to stick with newborn teats, rather than age-appropriate teats.
Do talk to some breastfeeding counsellors - NCT, LLL, ABM etc. Although Mumsnet can give good support, we can't give you the face-to-face help that can make all the difference.
Be aware also that the growth charts are not always used properly, and HVs can obsess over the line rather than the relationship of the line to the baby. Firstly, what sort of build are your family? If you are small people, or finely-built, then it would be ridiculous to expect your child to be big. Secondly, birthweight is not a predictor of growth. All my children were born at least 75th centile or over, and trolleyed along on 50th centile when allowed to do their own thing.
Fenugreek: IIRC the appropriate dose for me was about 6 capsules daily. But test it out on yourself - we're all individuals, and I'm quite a large one - by increasing the dose every few days, starting at about 3/day, until you (your perspiration and wee) smell of maple syrup. The correct dose is the minimum that it takes to maintain that smell.
Apparently it's even more effective if taken together with Blessed Thistle, but I haven't taken BT, so don't know anything about it.
BTW, fenugreek is mildly laxative and has to be taken with caution if you're diabetic as I think does something to blood sugar.
These are good breastfeeding websites, with info on fenugreek:
Kellymom
Breastfeeding Online