Your computer screen should be able to give you a pretty good idea of what the pics should look like printed. Not only that, but since you had images from 3 separate cameras ALL come out dark, I would think it's the printing, not the cameras.
I took some BW pics in to be printed. They all came out green. The minimum wage person saw no problem, tried to tell me that's how BW always looks. Oy. I literally had to go get a standard BW pic to hold up next to it, AND to get the manager before the problem was fixed. Turned out they were low on inks... but neither the employee, or the manager knew how to fix it. A whole separate person had to be called in (who understood digital photos AND the machine), who glanced at the photo, said "Whoa, that's green... betcha the ink is low" and poof problem fixed in under 30 seconds.
Costco is usually a decent place to get photos printed. For one thing they use high quality paper BUT if they're staffed with non-photo employees (or badly trained ones) you're up the creek. More than likely, the machine is set to print low, because most people overexpose, and that they don't/didn't run a test first.
What *I* would do is this:
- Take the card (and the dark photos) into a camera store (not a bestbuy/target/whatever multistore, but a camera store). They'll be able to tell you in a heartbeat whether it's the printing or the settings on your cameras that are causing the problems.
- If it's the camera... that's one of the beauties of digital... it's REALLY easy (although tedious) to adjust the exposure of the photos you've already taken on your computer using a photoediting program. You're in great luck, because even if you don't own Adobe or Aperture many camera stores have a kiosk where you can edit in shop using one of those programs for free, or if you'd rather, you can pay them to do it. The photoshop people will also be able to help you reset your cameras & to learn the programs for editing.
- If it's the printing:
a) get your money back
b) print elsewhere
You'll probably need to get at least 10 photos printed elsewhere FIRST so you can show the manager at Costco in order to get a refund.
As to working with the pictures before printing... for many people that's the "fun" part. :) Me included. Cropping, color, tone, effects... but I've never had pics that I HAD to mess with first... just chose to. Plus the money saving part. I only take around 5,###-###-#### photos a year... but I only "keep" 1 in 10, and I only print one in 50, and I only get one really really GOOD shot about 1 in every 100 (aka those go on the wall, the other printed ones go in an album). If I printed each and every single one I may as well be working in film. Digital lets me take 200 photos at a soccer game, or 40 of my son building the fence. I may keep only a couple... but for amateur me... it gives me the chance of getting those great keeper shots, without wasting thousands of dollars in film, and countless lost opportunities learning how to get them.
One of my best friends who IS a professional photographer can easily go through 30-100 rolls of film a day if she's working on a commercial project, 15-50 on a wedding or other personal thing. But she's being PAID to. I'd have to pay out of pocket, and OUCH. Huzzah for digital. If I had to pay... well I'd take photos like MY mum did. One or two shots here and there, and cross my fingers.