How to get a good supply:
Feed often. Very, very often (DC1, every 45minutes). Every time they open their mouth out your boob in it. Seriously.
Learn early feeding cues- opening mouth, licking lips, pushing tongue out, turning their head. Feed them. Don't wait for a set time or for them to make a noise or cry. Yes, this will mean taking seemingly happy baby off visitors - don't be put off by this.
In the evening (witching hour) when they are crying and grabbing at you. When they seem ravenous and scream at your boob. When they beat their tiny fists on your chest and won't keep your boob in their mouth, JUST KEEP FEEDING. It's really tempting to give a bottle then, for that to be their regular bottle slot but it's the time when your body is planning the longer term milk production. See it as baby putting in its order for the coming days and weeks - give formula at that time and the order won't be ready on time. By all means give a regular bottle to prevent bottle refusal, but make it earlier in the day.
Honestly, best advice: just keep feeding. As long as baby is wearing and has a moist mouth, you are making enough milk. Breastfed babies don't poop very often, by 6weeks mine were only going once a week.
Breastfed babies lose more of their birth weight than formula fed babies - health visitors/ midwives are trained to recommend top ups at a 10% loss but this is based on a study of formula fed babies, breastfed babies can be fine at 12% as long as they are weeing and have a moist mouth - dry mouth = dehydration.
With regards to the 3yo - I learned to feed sat cross legged on the floor. I'd like baby on a pillow on my knee and feed, keeping both hands free so I could read or play with my eldest. He was happy I was on the floor with him. I'd usually just build a simple 4 block Duplo thing, parallel play style and he was happy with that.
As for visitors - just make sure you feed baby when they need it, not when the visitor thinks they need it. Crying is the last feeding cue.