Hi MrsKoala
I strongly suspect DS3 (nearly 4) has ASD/SPD-I have an appt for him to see a paed on Saturday morning, for an assessment.
You so poignantly describe in your post of 13.06 how I've been feeling about DS 
In terms of balancing the other DCs needs, I just do the best I can. DH and I sometimes at the weekends take either DS2 or DS3 one to one so they both get a chance to do something special they want/choose to do and get full attention from one of us. DS2 and DS3 used to dislike being apart but that's recently changed so that's working out well.
DS1 is much older (14) and is stoic and pragmatic about things mostly. I know it's hard for him though, as we are limited as to what we can do as a family, but we muddle along, do stuff in the evening sometimes when the little ones are in bed, he can do stuff with his mates independently now, has a weekend job etc and would probably spend lots of time in his room/with his mates rather than us at his age anyway I think. He's very accepting of things as they are and mostly unresentful as far as I can tell. DS1 said tonight that although things are often manic and stressful here, it's also surprising, different and sometimes fun living with DS3 
DS2 (5) is a very gentle and joyful soul, and seems to have an incredible natural ability to defuse, distract, calm and manage DS3's behaviour most of the time. He has a neuromuscular condition that affects his hands and feet, but is affected mildly compared to many others with it, and DS3's energetic daredevil antics can be a good thing as they encourage DS2 to push himself physically in a way he might not if DS3 was less active reckless. In lots of ways they create a funny sort of balance as they're so different, and they expose the other to different ways of looking at things/doing things.
I home ed DS2 and DS3, so we are able to spend lots of time outdoors which seems to really benefit and calm DS3 a lot ( and DS2 loves nature/outdoors) and have very few restrictions re being somewhere by/at a specific time so can be very flexible, which suits us well. Today has been tough as I've had to do a 5hr round trip with DS2 for treatment he needs in London, so I'm shattered, DS3 is out of sorts because of the change in routine, and I've been quietly and self indulgently weeping because I just don't know what I should be doing for the best for the boys, because I am exhausted by the daily stress, because I feel so ill equipped and in the dark about what DS needs, especially without a dx as yet. I have no real practical family support, but if I did, I think I'd be enlisting GPs to do some fun things with DS 1 and DS2 sometimes (DS3 only wants to be with me or DH) while I did something with DS3.