@coolguy123 Everyone's experience is different so obviously I cannot make you any promises for how it will all turn out for you. But what I can do is post my story here so you can hear something hopeful.
My daughter was almost 5 when our boys were born. I was terrified. I was physically miserable. I needed tremendous help with taking care of my daughter. By the end I couldn't do much more than play games where she brought things to my bed. During the pregnancy I not infrequently had breakdowns panicking that I had ruined our lives, or her life.
Life after their birth was like night and day, a thousand percent better than life during the pregnancy. A singleton would definitely have been easier, but thanks to the second time mom advantage, the twins together still weren't as hard as my daughter was. You will need a lot of organization, you will need equal (if not greater) contributions of work from your partner, and you will want to pull in all the other versions of help that you have available to you. But once you've done all that - it's surprisingly manageable. We have our rhythm, we live by our routines, and we get all the work done.
My daughter LOVES her brothers and they adore her. I had expected getting them all out the door to school/daycare in the morning to be screaming chaos, but when she comes into the babies' room to pick out their clothes and fawn all over them, and they get to smiling and giggling back at her, it's my favorite time of day.