Its really difficult to advise on subject combinations when Dc isn't sure about Uni course. It also makes a huge difference whether they are looking at one of the 'top 10' Universities or something a little less competitive.
It sounds from what you say that DC is more inclined to the Humanities than the Sciences so would be looking at this sort of Geography course at Uni rathger than a science based one.
Geography & Economics seems a good combination but as previously mentioned, few degree courses require A level Economics so if Dc is very good at another subject but you aren't sure how they would get on with Econoimcs then it might be better to go with the 'known' option.
I would check about whether Government & Politics is considered an suitable 3rd A level by the standard of Unis your DC would be looking at. It would probably be fine as a 4th but if they take maths AS & then drop it at A2 so are only left with the 3 A2s it might be that G/P is seen as too soft an option.
My advice on what you have said would be to look for something that widens the range a bit more. Unis might see too much overlap between Geography,Economics & G/P.
As to Maths A level - I would say that while like most A level subjects there is a big jump it is actually considered a reasonaby easy option & most 'real' mathematicians take at least AS & often A2 Further Maths as well.
Maths is taken in modules - the Core modules & then various options, including Statistics. Many students wont have done seperate Statistics GCSE so while those that have may have a bit of a head start it isn't a big deal.
If Dc is taking 3 other stong A levels they would have the option of dropping Maths after AS but if they did well at AS they could then get away with lower marks on the (harder) A2 papers & still get an A grade.
RE- A* grade at A level. You do not have to get an A grade in all the AS papers. You need to get an A (ie 80% average) over all papers & average 90% over the A2 papers.
Few students would be expecting to get A* in all their subjects so I wouldn't see a problem with taking Maths & 'only' getting an A