Hi, I don't know you from before as I only joined mumsnet last summer when I was pregnant with my dd, so I am very new to this and am not very up with the lingo , in fact this is my first ever post! I saw your post and although I am very inexperienced compared to you, it struck a chord with me...
I wanted to recommend something which helped us get our dd to sleep (more or less) through the night after 5 months of being up numerous times every night and nearly losing our minds through exhaustion.
My mum bought our copy for us and it is an online 'book' called the 'Sleep Sense Program'. Our dd is only 7 months old, but the program caters for all ages of children.
I was highly sceptical at first and didn't look at it properly for ages, then when I did look, I initially did all the things the author recommended EXCEPT remove the night time dummy and lo and behold when things went downhill again I revisited it and realised what I had to do. Sure enough, after a difficult but brief cold turkey period, she slept through without her dummy. This was something that I had begun to think was never going to happen. I expect your ds at 3yrs, doesn't have a dummy, but this is just one example of the sleep 'props' they advise you to remove.
I still refer to this deceptively simple program from time to time when we have the odd set back to make sure I am not getting into bad habits and I think it might be able to really help you. If you'd rather not buy it (i'm not exactly sure how much it is, but not fortunes I don't think), then let me just say that much of what is recommended revolves around the creation of a bed time routine which must remain the same every day and it deals realistically with the thorny issue of the amount of crying you might expect and how to deal with it.
I know that the circumstances of your sons sleep pattern going pear shaped are very different, but perhaps this can help you all regardless of the cause.
Good luck whatever you do and if you have to go through some tough times (controlled crying is really rough on mummy), remind yourself that you are trying to give him the ability to enjoy a good nights sleep and that will last him a lifetime (and save you and your familys sanity in the mean time ).