I put the baby behind the driver's seat, toddler (who was still rear facing) behind the passenger's seat, and the kindergartener in the middle (who could buckle himself).
I'm one who functions best on a schedule. When I had little littles as well as babysitting kiddos, I had four under four many days. Everyone got a diaper change or went potty at the same time. That way I never had to remember who was changed at what time. Everyone (even those who were early elementary) went potty before we left the house for anything. It minimized the "I have to go potty NOW!" emergencies when we were out and about. Anytime we needed to be somewhere at a specific time, I would start "getting ready to go" an hour before, first with feeding the baby and making sure no one else would be hungry, then potty and shoes/coats. It usually did take just about an hour to get out the door.
I learned how to nurse sitting cross-legged on the playroom rug. Baby would eat, toddler(s) would sit next to me and I would read books, play matching games, build block towers, etc.
Babywearing made things so much easier. All my children have intense personalities, and my oldest was by far my neediest baby. The only way he was content was either nursing or strapped to me. You can choose to alternate who you wear, so the toddler can feel close as well. I also made a big deal about needing "big brother/big sister" help. They folded a lot of wash cloths and diapers, and matched a lot of socks. They stirred things while sitting on my kitchen counter. They ran and fetched things I needed (and often things I didn't necessarily need!). We put on a lot of music and just danced, since that helped relieve a lot of stress for everyone - not just children's music, lots of oldies too!
For my own sanity, we got out of the house often: library, grocery store, playgrounds, community center gym. Even just a quick walk around the block helped.
My children don't typically sleep through the night until about three and a half YEARS of age. Sleep deprivation just became a way of life, and you learn to function even when extremely tired. I wrote down many things in notebooks, simply because I couldn't trust myself not to get distracted or simply forget.
Simple, quick meals. Things that aren't done at a certain time are great: crockpot meals, pot roast, stew that can simmer for a while. You can start the meal when you have a free five minutes earlier in the day, and you don't have to stress about what's for dinner at 4:00 p.m. Meal planning is also helpful. I don't always plan each meal, but in our house we have a general schedule. Mondays are soup nights, Tuesdays are meat and potatoes, Wednesdays are some form of chicken and rice, etc.
It will go by so fast. And you will survive! Someone said to look at your day as "just twenty minutes". When the baby is hungry, the toddler is screaming, the cat just had a hairball, and someone left the bathroom faucet running..in twenty minutes, you can get all of that dealt with and things will be calmer.