I exclusively expressed for my daughter until she was about 9 months, by which point I had enough in the freezer to keep her going until 1 year, plus some to donate as well. She is just over a year now and is now taking cows milk.
In the early weeks you really do need to be expressing at least 8 times a day, including at least once in the night (between 1am and 5am). Over the weeks, you can gradually start to cut down a pump providing your supply is remaining stable. My supply was fairly steady until around 6 months or so, when her milk intake reduced due to weaning, so I wasn't worried about dropping more pumps and reducing my supply a bit. By the end, I was only expressing twice a day and that was enough to cover what she needed each day (plus I had my freezer supply to fall back on).
I would massively recommend investing in a double pump if you can. I bought an Ameda Lactaline from ebay for £40, it has a closed system, so no chance of contamination from a previous user. Really good investment for me, it cut my expressing time down by more than half.
You can store in the fridge for around 6 days (put in the back of the fridge, not the door), and the freezer for 6 months (proper freezer, not just a compartment in a fridge). I used to have all bottles for the day stored in the fridge, and I would keep rotating through, then putting anything left over into the freezer.
Around 12 weeks is when they say that your supply stabilises and is well established, so you can start to pump less (feed less) and still maintain a good milk supply.
Quantities per feed will vary massively for each baby. Mine, for example, settled nicely (of her own accord) into a pattern of feeding every 3 hours or so at around that age (I think - it's a bit of a blur!), and I think she took around 120-150ml (4-5oz) per feed. As she got older and dropped feeds, I actually increased what was in her bottles a little. She now has about 160-180ml twice a day.
I would say with a 3 year old and a baby, it would be hard work to exclusively express. Not impossible, but definitely very hard. I found it hard enough with just the baby, but I know other people have managed it.
Also, please consider how well you have done to breastfeed for these 10 weeks, despite things being so difficult! Well done and congratulations!