I had 3 in quick succession and didn't think about it a great deal beforehand tbh - we had agreed that we would like 3 and we didn't want big age gaps. There are big positives - they are very close, they play together, we have lots of fun, we can just about play rounders (important!), they are wonderful, 3 times the pride, love and joy.............I definitely wouldn't change anything, dc3 has brought a lot of happiness with him and the thought of not having him is appalling (obviously I feel like this about dcs 1 and 2 as well, but focusing on 3 as he is the one we wouldn't have had if we'd stopped at 2).
It can be very hard at times and it is very expensive - you don't really realise about the latter until quite a bit later on, so anyone saying that it isn't hasn't reached that part yet!
I had 3 under 3 and I managed to get out and about with dcs 1 and 2, but struggled a lot more once we had dc3 - dc2 particularly was a very difficult toddler and I found myself in some very tricky situations trying to handle her, plus dc1 (easier, but still very little himself) and a baby in a pram/sling or who needed feeding/consoling (reflux). We spent a lot more time at home and they watched more tv than I would have liked. Some of this may well be down to age gaps, rather than simply number of dc though!
We needed a huge car and still do really, though it isn't quite so huge (swapped a sharan for a touran), as they were all in car seats until very recently. You still need a big car once they start coming out of car seats, as they get bigger and need more space, plus a big boot for all the luggage when you go away (and for your golden retriever in our case). If you want them to have a bedroom each then you need at least 4 beds, or 5 if you want a spare, which in our case meant a significant compromise on garden size - we started out in a 2 bed cottage where they all shared, but obviously this couldn't be a long term home.
It can be awkward to find suitable holiday accommodation, particularly in hotels. We have had to choose between a limited selection of hotels that can accommodate 5 in a room or separating into 2 rooms. The latter isn't such a bad option now they are a bit older (especially as the split is naturally 1 with me and 2 with dh, as we have 2 ds and 1 dd), but it is very expensive! There is the extra airfare too - we are creeping up towards having to pay the higher fare for the dc (2 years left before the eldest is 12) so are trying to do more long haul now before it gets even more expensive! We love to travel and still do, but there is clearly a huge cost difference between travel with 2dc and travel with 3dc - I've never added it up and don't plan to!
Ours go to a private school, so obviously that's another set of fees, plus everything else you have to pay for, activities and so on. Even if you don't go private, you have to triple everything - our swimming bill in particular makes me wince every time I pay it. Schools shoes x3, trainers for school x3, trainers for home x3, wellies x3.........and they grow and grow! They also lose things, which drives me mad, especially flipping school blazers! £15 each for a new water bottle last term and one of them is lost within the first week. Forgot about the music lessons x5 (an instrument each, plus singing for 2 of them). It goes on and on. We are fairly well off and definitely feel the strain at times - we do lead a fairly middle class lifestyle with travel, private school, music lessons, etc, none of which is strictly necessary, but you do need to think carefully about the lifestyle you want and multiply everything by 3, rather than 2 - it adds up significantly!
Making sure all needs are met and all have individual time with me/dh can be challenging - we try very hard and it can be draining at times. I haven't gone back to work, partly so I can go to everything I need to at school without any trouble, drop them off, pick them up, ferry them around to all their activities, help with homework, etc, and also to try and make the most of the time I have with them - it already feels very rushed and chaotic during term time, adding work for me feels like too much. I miss working sometimes - career is definitely something to consider carefully when thinking about adding to your family.
Lastly, I don't think it has been brilliant for my health - 3 pregnancies in quick succession (and c sections), with bf into each subsequent pregnancy too and to 22 months with dc3. It was very hard indeed on my body and I am still trying to get back on track (youngest nearly 7). Also, mental health - occasionally I am quite stressed, sometimes I am anxious, often I am exhausted.
I sometimes look wistfully at families with 2dc and think how much easier it could have been, BUT I would never change it. I've been very honest about the hardships above, but it doesn't feel like that everyday, in fact most of the time it fantastic and full of fun. Good luck with your decision op - I do think that once you have 2 adding to your family is a leap of faith, it's not sensible really, but I do find it very rewarding (most of the time).