I’m sorry it’s been a struggle for you. Out of 40, having 5 die us a bit high, but not unusual. The chicks were shipped, and you may have gotten an unhealthy one or two that were more impacted by shipping or that had a defect that would cause them to die anyway. . The feed store likely just dumps them into their bins to eat and drink, but some take awhile to learn or are timid and dont get in to get food or water enough. You don’t know how long they were out for shipping, do you? If they shipped and arrived after 3 days, they were stressed and likely chilled.
your 3 heat lamps are only a problem if the chicks cannot get away from the heat and self regulate. So, if it is 100F under the two lights, but the chicks can move into a 90-95F area, then it’s fine, and not the cause of the issues. If they are unable to get away from the heat or all 40 cannot keep away from the 100F area, that may have caused some of the issues.
pasty butt, when poop sticks to the fluff and then dried and covers the vent. They cannot poop again because the dried poop essentially seals off the vent. This will cause them to die. So, under warm water, vent area only, gently soften the dried poop and rinse it off. Immediately put them into a warm towel/washcloth/paper towel and blot them dry. Since they are brooded outside, you may want to let them fully dry inside and warm up before returning them to the outdoor brooder.
mid you have not offered electrolytes, you might want to get a pack and offer them some electrolyte solution (replace every 24 hours or sooner). ALWAYS offer plain fresh water at the same time when offering additive water any kind.
if you have any liquid chicken vitamins (like Nutri drench) this is good to offer. If you cannot find that, then some people use baby Poly-Vi-Sol (no iron) to baby chicks. We give Nutri Drench to them straight using a toothpick to get a drop , and they will peck the dark drop or we touch the side of their beak and they naturally open it. Then we take them and dip their beak into fresh water so they take a drink or two. Give a small drop of vitamins 1-2x per day for 3 days, and they should perk up.
Good Luck!