First off, welcome to the CT forum...
Couple of questions... Were these bottle calves? Or calves on cows? If they were on cows, WHY did you wean them at 3 months? The cow's job is to feed the calf to get it to grow to be weaned at an average 7 months... Unless there is extreme conditions, and sometimes drought will be one, there is NO REASON to wean at 3 months under normal conditions. And then, if you do wean off cows at that early age, they need to be fed well, so they will keep growing.
You said that there were several reasons they were at lighter weights and looking a little rough, and admitted it was more your own neglect/fault. I admire someone that can come right out and say, Hey I didn't do things right... need to try to fix it, make it better, and have learned from it. SO kudos to you for that.
If they were bottle calves then 3 months is good for weaning but they need good high quality feed and real good hay to keep on growing for at least another 4-6 months.... or they will be stunted.... By the time they hit 7-8 months, they still need good feed/grass, but they will have gotten a good start, and good grass will go a long way to helping them grow better with less concentrated feed.
The guys from Canada will have bigger cattle at weaning that many of us here in the central, eastern area.... VA, NC, SC, TN, KY.... many of our cows are not as big as the farther northern cattle and do not wean big calves like they do... Most of what we wean is in the 450-600 range... at 7-8 months... a round figure here is 75 lbs at birth, 50 lbs mo gain... and we try for better than that... but that is a very "round figure guestimate"... Has some to do with the humidity, crazy weather swings we get, the soil and grasses they have to grow on, age of weaning too... and the smaller size of the cows overall... ours run in the 1050-1250 range. We don't push them with feed, but they learn what some silage is, and once weaned they get silage and some grain top dressed, and good hay.... plus grass when we have it for them. They will get this for 60 days or more, and then we normally sell them as feeders. The heifers we keep for replacements will get some grain at pasture, mostly for the "keep them coming to the catch pens" a couple times a week... but they don't go out until they are pretty good size.
Fecal samples to determine worm loads... no need to worm too often, but timely worming can make a big difference.
Salt and mineral should be kept in front of them all the time