Mission: Impossible Possible?

I'm seeking a new diaper bag, and it seems like an impossible mission!

I still like this bag I got last year at TJ Maxx that could double as a purse and diaper bag and was cheaper than any Walmart options. I adore it, and actually still carry it as my everyday bag, but now I need to find one for the needs of 3 kids.

I must find one that will hold:

*all the needs of a new/growing baby (diapers, wipes, creams, pacifiers, & eventually jar foods & sippy cups)
*drinks and diapers for Josiah (until he's potty trained)
*drinks & snacks for Kyleigh (she will not outgrow the allergies)
*my wallet, keys, BlackBerry, sunglasses, chapstick, etc
(I have no intention of toting a diaper bag for 3, plus 3 kids, plus a stroller, plus a PuRsE...nope!)

I've heard great things about Petunia Picklebottom bags, but no personal experience with them...anyone got feedback on those or how they work out for use w/ more than one child? When I Googled "Best diaper bags for multiple children" I got this site. I don't care about name brands or anything...just about the function & value for the cost. That's why I'm searching so early, in case this will be an "investment" piece, I want to have time to plan and save up! I'm really hoping for some of you mommies to let me know your diaper bag tips! Thanks ladies!