i figured out what they are, elm seedlings from a nearby large elm tree.

So, how do i get rid of them? prefer something environmentally friendly and yet not hands-and-knees manual removal.

Mowing will probably take care of most of them. Trees want to be taller than 3"! They may remain and try for the rest of the season but will not live if constantly mowed. Any product that kills dandelions should also take care of them.
hortster

Proper mowing will eliminate any tree seedlings that do start to grow. Once the leaves are removed, from most tree seedlings, they can no longer supply needed nutrients to the roots so they die.

Mowing is not really working for me..! I've had these for two seasons now, and I mow usually twice/week. These dont rally get very high, there are 2 or 3 sets of leaves below the height of the grass.

In some 50 years of gardening I have not seen an Oak, Maple, Elm (American or Chinese) seedling that could not be controlled just be mowing. It may take a while, but once the leaves are cut off the root will die and the seedling will be no more.

Ah one of the joys of living with mature trees!

I would learn to embrace these little sapplings if I were you because as long as there is an elm tree (or mountain ash, maple, or most other mature trees), you are going to get sapplings. You will have to continually pour herbicide on your lawn to control them and the lawn will give up the ghost first from the toxicity.

You might want to look into applying a pre-emergent herbicide on your lawn that will prevent the seeds from germinating. Corn gluten is out there, possibly others. I'm not familiar with US brands, but maybe someone more knowledgeable will chime in.

I'm guessing if you don't want to pull them (pulling is most effective when sapplings have 3 sets of leaves or more), you are not going to want to sweep your lawn either. Lawn purists for generations have used Besom Brooms to sweep debris off their lawns prior to mowing (no, I'm not kidding!)