From what I read:
Beguiling Voice - You can convince someone you are right about something, but it doesn't make them do anything. You will still have to convince them of that, but are likely to get a bonus. Convincing someone that letting you into the castle might be a good idea, but if that guard knows what will happen to him from his superiors if they find out (or you once they open the door), getting him to open the door might still be a problem, though less of one after the spell. The spell might just be the only thing that even makes this possible, at all; otherwise there is no chance of getting the guard to open the door.
Command - Is just what it says it is; you give the individual a direct command, though it cannot be the antithesis of their beliefs or well-being. Pretty straight forward. If the guard (above) believe they or their lord will be killed/harmed if they let you in, then the spell should not work. If they believe that letting you in will not harm themselves or their lord (or anyone they truly care about, really), then the spell should work. Anything short of direct harm will be done, it is the most powerful and direct of the 3 spells. I would say, due to the spells power, that even something that could be harmful to the target might be obeyed, provided the spellcaster is high enough level (beyond the target) and the target whiffs their RR (by a large margin). So, a high level evil mage, can tell the lowly man-at-arms to poke his own eye out, it just takes a large level discrepancy and a poor RR. But that is just my own evil take on the spell.
Suggestion - To me, this spell is just the uber-subtle version of command, though it does have the added trouble of needing the spellcaster to have a reasoned argument/idea. Ultimately, it is the sneaky characters method of accomplishing a goal while leaving very little evidence behind, and possibly leaving the blame on someone else. ("Thoughtful" and "subtle" being the key words in the spells description.) It also has the added benefit of a much greater "duration" than the others; as long as it takes for the suggested action to accomplish. So, if you find the guard the night before you need to get into the castle, you can work the spell on them then, and if successful, they will perform the action when they are next on duty, even if that is days away. Very nice.