I had 8.5, 3 and newborn. I would never have planned those age gaps but we had fertility issues and had always wanted three.
There has never been any jealousy with ours, DC1 tends to do his own thing and not spend much time with the younger two but I can never work out how much of that is the age gap, how much individual personalities and how much is impacted by his autism. Younger two are really close.
The baby stage with DC3 was fine. It got a lot harder once he was a toddler and eldest was a preteen. Probably at its absolute hardest when they were 14, 7 and 3. Got easier once DC3 was at school and has continued to get easier ever since.
It is undeniably hard juggling the very different needs and stages and I do feel that at times DC1 missed out because, as others have said, we often had to plan activities based on what the youngest could do. But there was never any major resentment - we just had to divide and conquer a lot, DH with one, me with the other two.
Positives: you are so much more relaxed with DC3 because you're so experienced. I always felt that the baby days were like being a first time mum with loads of time with the baby (because older ones were at school/preschool) but none of the stress. I can genuinely say that I have enjoyed the early days with all three of mine because I was never frantically juggling tiny babies and very young toddlers. I love the dynamics of having three, even though you do realise that the world is set up for families with only two! Both DC1 and DC2 have matured into sensible considerate teens and I'm sure that is partly because they both had the experience of being older brothers, whilst DC3 has definitely benefited from having those older brothers, too.
Negatives - there are definite moments of weariness at going through each educational stage again, third time round (can vividly remember thinking "Christ, I can't believe I'm still on fucking 'Big Cat'!" when trying to simultaneously do early reading practice with DC3 and help DC1 with an essay on Macbeth). I can only imagine how over it I'm going to be when poor DC3 is into the GCSE/A level/UCAS years, having done it all twice before...
That said, though, a surprising positive when you have big age gaps is never really feeling sad and nostalgic at leaving certain stages behind - mainly because you've been doing them for so bloody long that you're delighted to finally ditch them (i.e. I read a bedtime story every night for 16 years and God, the absolute JOY of not having to do that now DC3 is 10 is indescribable. Whereas I have friends with classic '2 with a 2 year age gap' family who still kind of mourn that loss. No mourning going on here, I was delighted, I can tell you 
).
Overall having three with larger gaps is far more positive than negative but I would caution anyone against having a third child because they're keen to experience the baby days again. You have to want to have three children, three teenagers, three actual PEOPLE to parent in your family. The baby days are gone in the blink of an eye!