Firstly Puppymum. It can be such a difficult time, and so hard on you, when there not sleeping.
At 3 months of age he should only really be awake for an hour - hour and a half at a time, including the feed. It sounds like he is very over tired. It can be very hard to break the no sleeping cycle, but its prob best if you get him to sleep by any means at this stage eg. a sling or baby carrier, the pram, car, with you etc. Sleep often promotes sleep iykwim? Then you can focus more on other settling techniques.
I hope things get better for you.