Mossie - there is absolutely no need to give formula in a cup. If you feel that you need to do it, then that is something else but it doesn't sound as if you do.
My first baby NEVER came off the breast himself - I had to decided enough was enough and break his suction with my pinky.
When he is on the breast is he actually sucking all the time or sleeping with the nipple in his mouth?
When he first latches on he will suck vigorously till the milk lets down then he will suck more slowly. He should swallow after every suck which shows he is getting milk. Listen to him as he sucks and you should hear it. He will pause every so often as the milk is let down in separate jets. Watch that the pause does not last too long - he may have fallen asleep. Tickle his cheeck or blow gently on his face to get him sucking again. As he reaches the end of the feed you will see him 'flutter suck'. Do not take him off at this stage as he is actually getting some hind milk. After about 10 mins of that, I would take him off, wind him and offer him the same side again. This will ensure he is getting the richer hindmilk.
As has been said before though, there isn't a clear demarcation line between fore and hind milk - it merges in together. The same process should occur. Once he is off the breast again, if he is looking for more, put him on the other side.
Giving formula just now is not necessary and it can have an impact on your milk production. You just need a boost of confidence.
Are your nipples OK? Does it hurt to feed? Is his head and body in a straight line when you attach him? Do his cheeks look full when he is on? It will be difficult to see curled back lips so cheek fullness is a good sign he is on properly. Is his chin up against your breast? All these signs show if the baby is on as well as he could be.
A baby who is not attached really well will take longer to feed. Will post a link of clips in a minute.
Runny, ellow poos are normal. Please do not worry about your diest at the moment - it does not sound as though there is intolerance here.
You and your baby have only been breastfeeding for just over 3 weeks - it is still getting established. You will get there