Okay, it is pretty simple when you break it into 3 cakes.
It looks like she has a 2 inch tin size difference. I would go for a 6, 8, 10 cake sizes. They look pretty high so possibly 3 layers in each cake. Do they want all sponge?
Cake one - 10 inches, bake fill, dirty ice, fondant cover, measure out the circumference of the cake so you can work out how many semi-circles you can fit around the bottom. Cover a 14 inch square board and set cake in centre. I would use a sausage of fondant or royal icing rather than piped buttercream.
Cake 2 - 8 inches - same as cake 1 but with circles/anchors, set cake on a 8inch thin card.
Cake 3 as for the other 2 again set on a 6 in board. Decorate with stripes etc. but don't place topper.
Find the centre of the bottom tier and place your first dowel (push the whole dowel into the cake, make a mark where it meets the icing, remove and cut at the mark. Cut more dowels to the same length as the first one.) place them north, south, east and west of the initial dowel, about 2.5 inches from the centre. These 5 should hold the cake but if you need the reassurance place another 4 in the gaps. Do the same for the top of cake 2 but place the first 4 dowels 1.5-2 inches from the centre, again you can add extra if you feel you want to be extra safe.
Stack cakes on the day at the venue and hey presto.
You can do it :)