3rd time not so lucky...

so I have 3 boys aged 3 years, 1 year and 4 weeks.

my 1st and 2nd were good sleepers as in we gave them their last feed and that would be them down for 8-12 hours each night...lucky I know!

my 4 week old won't settle! We haven't slept in 4 weeks I'm at ends wit now and I don't know what to do! He will sleep on me or my partner but as soon as we put him down he screams and now my 1 year old has started waking up each night as well!

we have tried everything from warm baths to cot mobiles and nothing works! 

is there any thing else we can do to try and get him into a pattern of sleep at night time? 

its starting to cause agro in the house between me and my partner now....and we have been having more fall outs, I snap at him because his snapping at the kids it's a evil cycle and I can't see any light at the end of the tunnel