it is pretty much the consensus to rear face AS LONG AS POSSIBLE. So whle it may only be the law to rear face until 12 months or something, shop for a car seat that rear faces much longer if possible. Like, 3-4 years old if possible. SO much safer to rear face as long as possible... (Look up youtube for good examples of WHY extended rear facing is the best option. Caleb is about 9kg now; there is NO WAY in hell that he is forward facing for years yet!) And then forward facing in a harness, then into a booster seat until they are tall enough for the seatbelt to sit correctly on them (often about 8-10 years old).
Most good convertible car seats can go from birth right through to a booster in the one seat, so you could get away with one seat per child that way. A capsule can be handy for a newborn though as they can be carried in the capsule!
So if I was you I'd get a capsule initially; buy it as late in your pregnancy as possible though, and get it brand new. They generally have 6 years until they expire, so you might just be able to use it for all three. Then you can get a convertible seat for each one as they grow out of it (and keep the capsule for the next one!)
Alternatively you do your research now and go straight to a good convertible seat to last the whole time, and skip the capsule totally.
Yes, there are good convertible seats that will fit three-across the back seat (including good extended-rear-facing ones), but you will need to research that yourself as the brands you have there will vary to here!! Here the 'radian' is great; rear faces till like 45lb, and lasts right through to a booster seat. And fits three across in most average cars!! https://ethertonphotography.blogspot.co.nz/2010/08/sunshine-kids-radian-xtsl-car-seat.html But if you research them specifically for fitting three across over there, then you should find some to choose from; there are heaps of people with three kids in car seats and only average cars!!
Here is a good thread: https://www.car-seat.org/showthread.php?t=33226
Oh, and research shows the safest place is the middle of the back seat, next safest is passenger side of the back seat, then drivers side of the back, then passenger front.