I would be hesitant to put herbicide down on the sod being so new, especially being warm season turf put down in winter. After it gets rooted and mowed twice, I would use an ester based herbicide if it has not died with the warmer temps. by then.
Oh sh!# I'm sorry just realized the thread said 3 years not weeks, oops, I would use an ester based herb. maybe Turflon, MCPA, something in that category, being how there is little leaf surface area I would also use a spreader sticker to help adhere it to the leaf area.
Your wild onions have a very waxy coating. If you can mow em, kick em, skuff em, do anything ya can to disrupt that waxy surface your herbicide will be more effective. Ya may want to take the cameras away from your wife and neighbors prior to your pre herbicide activities. The mental pictures will be enough to entertain most people.