If a 3 YO LH heifer hasn't ever been bred (or exposed to a bull) then she probably is fertile. Only way to tell for sure is to expose her to a bull and see what happens.
A 3 YO should probably weigh between about 900 and 1100 lbs., preferably between about 900 and 1000. LH tend to weigh less than "commercial" cattle. We have one 1200 lb LH mature cow; however, she has a large frame and is about 57" tall at hip.
If she is bred, then have her ultrasounded or palpated about 3 months after estimated service date.
If your 3 YO LH is much over 1000 lbs., then I'd put her on pasture/plain hay ration to get some weight off of her. If she is overweight, do not feed any supplements or high % hay. Of course, make sure she has a quality mineral tub, salt, fresh water. Most of our producing LH cows range between about 875 and 1000# and do quite well. Our 1st calf heifers that are first bred usually weigh between about 650 and 750 at about 12-14 months of age. Just be sure to use a "small" bull with 1st calf heifers so he doesn't put too much weight on her back when mounting.
