well I am slightly biased as I have 5, but I would say.......GO FOR IT!
for me, baby no.3 was the easiest baby to slot into our family. DD1 then was nearly 5 and DD2 was nearly 3 and both were at nursery/reception at the local school. it meant that baby was already in tune with the routines of the family and as I had already 'done it' twice befoer, I knew the basics. (well, nothing is quite the same, BYKWIM).
car wise, we had a normal estate car and didn't need bigger (that came with the next pregnancy), and as for sharing rooms, the girls already were so that wasn't a problem either. we had a room for me to set up as a nursery and I loved it......especially as we had had the older girls in a small 2 bed flat and this was our new 3 bed house. (yes, we did the new house/new baby thing!!!!).
money wise, we still had a lot of equipment. both the girls were out of nappies and also of an age to 'help', plus they were out of the house for at least half the day each and so the third baby had time with just me (incidently, we had a third daughter so was even easier re clothes etc).
I would say to anyone........the third is the easiest, especially if it the at least the same sex as one of them before. for e, the hardest and most shocking was the second....not least because my expectations were blown out of the water. (I had nannied twins for five years before becoming a mum and thought it would be the same thing.......IT SOOOOOOOOOOO WASN'T!)
OOh, the next biggest shock for me, and this is only my shock again, was having a boy for my 4th baby......oh my god, what was that all about.....willies and all that. .
if you want it, go for it, all you will always wonder 'what if'.
hth