When you're saying what's best or giving advice, use should: You should tell him what you think.
When you're imagining a different situation, use would: I would take the earlier train.
When you're suggesting an option that isn't fixed, use could: We could stay an extra day.
In short: should recommends, would imagines, could offers a possibility.
If you're weighing options and nothing is fixed yet, use could for present or future possibility. We could move the meeting to Friday. This signals you're not certain, not that you're recommending it.
If you're describing a skill you once had, use could for general past ability. I could swim before I turned five. This works for repeated or lasting skills, not one single past event.
If you're imagining a different situation, use would in the main part of a conditional sentence. If I had more time, I would learn French. You're picturing something unreal, not suggesting it should happen.
If you're recalling routines from the past, use would for repeated actions. Every summer, we would drive to the coast. This fits narratives and formal writing, and it needs an action, not a past state.
Contrast to watch: We would meet on Tuesday imagines a scenario, while we could meet on Tuesday offers it as an option and we should meet on Tuesday recommends it. The difference is what you mean, not the tense.
One exception to remember: with I and we, should can act as a formal stand-in for would in imagined situations. I should love to meet her again if I had a chance sounds old-fashioned and formal. The neutral choice stays the same meaning with would: I would love to meet her again if I had a chance.
This swap only works with first-person subjects. With you, he, she, or they, keep would: She would love to meet her again if she had a chance. If your sentence doesn't start with I or we, and you mean recommendation, plain should is still your word.
So the quick test: if you're writing I or we and want a formal, slightly dated tone, should can replace would in the imagined part. If that's not your situation, the usual split holds: should recommends, would imagines, could offers a possibility.