Left entirely to your own devices, what would you require to live for three days on your own?
Three changes of clothes. You can use civilian clothes or BDUs. There are summer and winter BDUs; Choose according to the season. I have two pairs of jeans and four tshirts (two white and two are OD green), plus six pair of underwear and eight pairs of socks. When your nuts and your feet hurt, it can take you from an efficient and effective state to a sniveling mess in no time. Take care of your feet and nuts. Keep them dry, clean and in clean clothes. I have the tshirts, socks and underwear in ziplock bags with the air squoze out of them. I have such an odd loadout due to weight and space, mostly. I am going to add black sweaters and long thermal underwear.
I have 160 rounds of rifle ammo on stripper clips on top the clothes, and on top of that is several packages of crackers. Then I have hiking shoes (some variant of combat boots is recommended) and a 4 D cell flashlight on top.
Three Mountain Home dehydrated dinners are in the inside pouch, along with a Dinty Moore meal and my hygiene kit. Don't buy rigid container toothpaste - get something that will take up less space as it gets used.
In the left outer pocket, I have a Gerber folding saw and Wells Lamont work gloves. Center pocket holds spare AA and C or D batteries and a scanner. Right pocket has insect repellant with DEET, SPF 45 sunscreen, 2 chemlights, a lensatic compass and a 2 AA cell flashlight.
In my SEAL link, it explains the three line principle. I don't have that yet. I don't have my 3rd line fully stocked. It is a start, however.