if u want to get an approximate 50mg line and you can split up the 3 grams into grams... take one gram, then split it into 2 equal sized lines/piles.. then take one half of that, and split it into 5 equal sized lines frmo there.... those should be ~100mg... then cut one of those in half.
or you could take the 2 lines you got from the gram, take one of those and split it into 2 more lines, you'll have 2 250mg lines... take one of those and half it into 2 lines, 125mg lines... then half again, and you got 2 62.5 mg lines.
its not exact but its as close as your gonna get without a scale. and if you dont have a scale to weigh grams... well then your not a proper druggie yet!... lol j/k... but if u dont have a scale but have some of those small amber jars, the normal ones, NOT the really short squat ones, usually half about .5g when you fill them up almost to the top... so you could always use that to get yourself down to 1/2 gram.
again not the most exact method... but considering nobody has any idea what the purity of their shit is anyway it doesnt erally make a difference if you were off by a bit and you split the gram into .6 and .4 however u split it and ended up using the .6g line to continue to break down by eyeballing, as the chances are your idea of the purity of it will probably be way off in accuracy compared to your accuracy eyeballing it as i described above so even if you had a scaleto measure teh shit you still wouldnt erally know how much u were doin