Get some 72mg nic before 20th May, Darkstar or Lucemill are well recommended. Nic shots will work out too expensive. 1 ltr of 72mg nic will make you nearly 25 ltrs of ejuice at 3mg for £30 (plus PG/VG & concentrates).
Recommended shopping list;
1ltr 72mg nic
2 ltr VG
500ml PG
concentrates of your choice, look at Oneshots to start with.
The calculator on here is great & easy to use.
Dilutents are the PG/VG (plus water/Vodka for thinner juices) or Aqueous VG.
Simple mix using Oneshot at 20% for 100ml @75vg/25pg
Nic 4ml (3mg strength you wanted)
PG 1ml
VG 75ml
Oneshot 20ml
Shake & steep (or vape if fruit/menthol)
For 80/20 mix you will need to buy nic in VG & source flavours also in VG.