You suggest a course of activity (limited to a sentence or two) and magically influence a creature you can see within range that can hear and understand you. Creatures that can't be charmed are immune to this effect. The suggestion must be worded in such a manner as to make the course of action sound reasonable. Asking the creature to stab itself, throw itself onto a spear, immolate itself, or do some other obviously harmful act ends the spell.
The target must make a Wisdom saving throw. On a failed save, it pursues the course of action you described to the best of its ability. The suggested course of action can continue for the entire duration. If the suggested activity can be completed in a shorter time, the spell ends when the subject finishes what it was asked to do.
You can also specify conditions that will trigger a special activity during the duration. For example, you might suggest that a knight give her warhorse to the first beggar she meets. If the condition isn't met before the spell expires, the activity isn't performed.
If you or any of your companions damage the target, the spell ends.
* - (a snake's tongue and either a bit of honeycomb or a drop of sweet oil)
So my players had someone powerful cast mass suggestion on them, he said 'i'll help you in your main quest if you give me the artefact you are going to find in that tomb'.
That artefact they search, they think it could help to cure a powerful ally. Attuning will show it is not the case.
Q1. Is the question reasonable or does the spell fail?
2 characters succeeded the save, 3 failed.
So they find it.
Q2. One character attunes which takes one hour, would the other characters wait, as attuning would make said request be more reasonable ? Or would they want to take item to caster ?
Q3. One who succeeded wants to make a failed character cast detect Magic on himself to see it is affected by a charm effect, so it casts dispel Magic, valid?
Q1: It's really up to you, but it seems reasonable to me.
Q2: It says they need to follow the suggestion "to the best of their ability", so I would say the characters that failed the save should not want to wait around for someone to attune to the item before they bring it back. That said, the character who's attuning could maybe try to persuade them that going back will be too dangerous if they don't rest up first.
Q3: Dispel Magic works automatically on spells of level 3 or lower, so it would automatically end this spell unless it was cast at a higher level. However, the spell you're talking about is actually a different (but similar) level 6 spell called Mass Suggestion. To remove that spell with Dispel Magic requires making an ability check with the caster's spellcasting ability, with a DC of 16 (10 + the spell level) as detailed in the Dispel Magic description.
It makes the saving throw have better chance roll d20 using wisdom throw after characters ended their turned for long rest.