The drop target itself needs to be replaced, there is a little lip that catches and holds it in the up position, they wear out/break after millions of hits. Any pinball store will have replacements, just make sure you order the correct ones.
The switch error means you have a bad opto or receiver in your trough. if you are real handy with an iron you can replace the opto and receiver for a few bucks, or replace both boards with the Great lakes modular replacement set for $70 I think. Again, Most pinball shops sell these

Yes, all three targets should start up, you need to knock them down to light the Idol Lock.
Go into the test mode and first test the opto switches on the drop target board. You will need the playfield propped up so you can reset the targets manually after knocking them down. There are three "U" optos on the target board when you drop the target the switch should register on the DMD and tell you what number just closed. If you don't get a report on any or all of them first try cleaning them, a little compressed air can and a Q-tip should do the trick. Then test them again. If they do not register reseat all the connectors on that board and the 10 opto board (it's the big square one on left side under the playfield) Test again. if no luck then you most likely have a bad drop target board (luckily replacement boards are available and fairly cheap) There are also 18 million other problems that could cause those switches from working, but currently we dont need to open that can of worms.

If the switches all register then go to the drop target test ( or coil test, I don't remember if there is a target test in the menu) find the coil that raises the drops and run that test, you will need to have the interlock closed to enable high power to the coils for them to fire. Watch the coil and reset bar they should push all the targets back up into place (except the broken one you know about) knock the targets back down and repeat a few times. You want to see if there is any binding on the reset mechinasim or if the targets them selves are catching on any thing.

Hope it helps, good luck!

Sort of, I run Buffalo Billiards, we have about 14 pins that I take care of plus a few of my own personal machines.

I believe they are supposed to reset after every lock. there may be a setting somewhere that allows you to change that. Unfortunately its been a long time since I played IJ.
Here is the POA Picture I mentioned in the message.

ij_poa with notes (resized).jpg