Congratulations! I know you don't feel it now... but you are on the brink of awesome...
I'm a bit further on than you... DT's are now 6 later this week, and I have an older DS. I found the pregnancy so incredibly tough that once they were here it was actually so much easier.
Everyone will think you're amazing just for existing and that's a huge boost. You can lower standards without accompanying guilt, because frankly, you're super human. Everyone is in awe of you just for the basics.
The biggest plus is their relationship. When you've had one already you think nothing can top those cute moments, but believe me, when they are done in pairs it is beyond worth it. Now they are older I'm so glad they have each other. They bicker occasionally but generally are 100% each others best friend. They've been capable of independent play from much younger and for much longer periods than my singleton every was because they have each other.
And what someone else said upthread about twins being sleepier proved true in our case... they were premie and slept 23 hours out of 24 to start with. they were on such a routine from hospital they just carried on. I kept thinking "when is this bubble going to burst?" and it didn't... by the time their sleeping requirements shrank they had started preschool so I still got a break...
The biggest difficulty I'm finding is the impishness now they are older... due to having a peer on exactly the same level there is quite a bit of naughtiness that goes a step further than a singleton ever would because they goad each other on... so we've had drawing on furniture age 5 for example... I also found it tricky when they were toddlers and I didn't have enough hands... DS always got the short straw then. But very early on I taught them the 'Stop Go' game with no second chances, so they were soon very ordered little ducklings following mum...
But it's brilliant overall. You will regularly feel superhuman just for surviving, and that's great. Also the dynamics of three are so incredibly rewarding, it's mad, it's full on, but you'll have a house filled with happy chaos and laughter. And you really do feel privileged and like you're part of this unique club who got something amazing...