This last week DD has started being OK with being left by herself for very short periods of time- she's 8 and a half months.
I am going to give you my opinion here but please remember that everyone parents in different ways so if this isn't for you just say and others will give you suggestions more up your street.
Personally, I would make sure he had attention all the time he was awake. If you don't live with a partner and/or have family nearby that will be hard. My feelings with DD were that she spent at least 4 months as a conscious being inside me and I'd give her at least that amount of time with constant cuddles if she wanted them. OH has been of the same opinion. DD has not wanted to be on her own until this week and so we haven't left her. She has been put down lot but there has always been someone in the room with her when she is awake. True, it's harder to do things. But my number one thing is her, not the housework, not my job, though both of these are very important.
I'm trying to think what I would do at 3 months. If I am right, she could play with things for between 10 and 30 minutes. So I could put her on her mat with the play arch above her (I adjusted it with ribbons so she couls reach the toys and that made a far better toy) for half an hour and talk to her as I did something else. Or put her in her bouncy chair for 10 minutes, again, talking to her (or singing) as I did other things. Then on her changing mat for 20 mins (I don't know why but she loves it!). Combining any lying down with nappy off time was a big hit too. though I wouldn't have done all these things in a row I'm sure you could if you need to.
I would recommend getting a sling. Not a Tomy or Bjorn as they would last you very little time but rather a decent sling that will still be supportive when he is a toddler. There is a babywearing sticky and many threads so putting 'sling' into the search bit should get you lots of advice.
OH works from home and he has often been looking after DD at the same time. He often puts her in a carrier (he prefers to use a connecta though he has just started using a Kanga x too. I like my carriers, we have lots
). He has also attached another monitor to his PC so he has a dual screen. I have no idea how you do that but he didn't need anything special to make it work. This means he can work on one screen and put on something from CBBies from iplayer on the other screen. Not all day, just for 15 minutes or so. Now she is more mobile and alert he can't work with her on his knee as she grabs and hits the keyboard.
ETA: I've been thinking about this and realistically unless you are a childminder or something similar I don't think you really can work at the same time as looking after LO. Yes, OH sometimes works while DD is awake but usually it is after I am home or when she is asleep - either nap time or in the evening. If you want to work during the day while LO is awake you will probably need to get someone to care for him while you work. I don't think it is realistic to expect him to occupy himself until he is much older and certainly not for many hours each day so you can work.