I'm no expert, but I think that it is pretty normal for them to "wake up" - ie become more alert after feeding, at around 3 weeks. You might also just be experiencing a couple of unsettled days whilst your son has his 3 week growth spurt.
It sounds like you're doing all the right things. I absolutely agree that you have to choose one method of settling that you are happy with and stick to it. I was also told to use the settling method to calm the baby, but always put them in the cot awake or drowsy. That way they learn to settle themselves without your endless assistance (particularly important during the night).
If you have been feeding to sleep, this is all your baby knows and might have to learn another method to settle himself down to sleep.
At 3 weeks, your baby is probably only happy to be awake for 1-1.5 hours, including feeding time. Once overtired, they are even harder to get to sleep.
Do you wrap or use a dummy? Wrapping is said to stop them throwing their arms around and is similar to being squashed in your womb.
I found the dummy a godsend and it didn't interfere with breastfeeding at all. I couldn't do without it. I just let my daughter have it to get to sleep, as it just seemed to calm her. I would take it out once she was asleep, as I didn't want her to become addicted to it all night.
You could try teaching your baby a routine - eg. feed, then play, then sleep,so he knows what to expect next. Breastfed babies might need the occassional, feed, play, feed, sleep, because they seem to get hungry so often (must be all that quickly digested breast milk). Of course at night the routine is just feed sleep.
Other settling methods might include:
* patting in the cot until calm then gradually stepping away, only come back if they get upset
* put him down, tell him to go to sleep, walk away. If he gets upset, pick him up calm him, put him down again. This might take a lot of work in the beginning, but as the days and weeks go by, it should take less work. (I saw this method on the Baby Whisperer on ATV World last Monday night, might be worth watching).
* in conjunction with one of the above methods, use sleep cues - ie a pre-sleep routine that tells your baby it is sleep time now. For us it was wrapping, dummy, cuddles and the same phrase repeated over and over again. Something like "it is time for sleep precious, good night".
This website also has some good tips on settling strategies:
http://www.swsahs.nsw.gov.au/karitane/docs/survival.asp
I was told to try settling for up to half an hour for a newborn. If that didn't work, give up and go for a walk, give a massage or a bath and start again when you see the tired signs.
Good luck and let us know how you progress.