How to fix stair?

must be replaced with a solid board ==== if you can get under the stairs then it can be reinforced from the bottom side .........the three pieces was a poor original install and should be fixed as it is a hazard that may go through and then you wind up with a ""broken leg"" == sounds cheaper to replace than a hospital bill and cruthes for six months and hiring a chauffer .... etc..... fix-it ... OK.....

look away

Install a 2 x 4 across under it.

Reinforce it

Your description is not sufficient to determine IF anything can be done, or even needs to be done. Without at least a photo, the only useful answer is to have someone LOOK at the step.

Call the handyman!!