2.75+.003+.158=
Make sure you "line up" the decimal points before you add.
2.75
0.003
0.158
What do you get?
I copied and pasted your problem into the Google search box --
2.75 + .003 + .158 = 2.91100
That first one could be written 2.750 if it makes it clearer to you.
2.911
To add these numbers together, you simply need to follow the basic rules of addition. Start by aligning the decimal points of the numbers:
2.750
+ 0.003
+ 0.158
Next, add the numbers in the rightmost column (the ones place):
2.750
+ 0.003
+ 0.158
------------------
2.911
Since the result has three decimal places, the final answer is 2.911.