I want to replace a dimmer switch, with a new one. my new dimmer switch has two black wires and one green. The wires inside my wall has three black wires. How do l connect them?

SOURCE: Dimmer Switch Installation issue

ALL you really need to worry about is the white and black wire that you hooked up before. The green ground screw is not needed. If you have a bare wire completely bare all the way, you can hook it to the green, but it is not required to work.
The other 2 screws are for the switch. It does not matter which wire goes where, as the switch just puts them together, and takes them apart.

SOURCE: I want to replace dimmer switch with standard switch

the two black wires spliced are Live wires ( same value as RED) the other single black is the return. The two twisted blacks from the wall go into ther active hole of the new switch and the single into the return. No third wire for earth is needed

SOURCE: I have two wall switches in a hall for an over

You cannot, at least not using the dimmer you have in your hand. The switch with all the wires is a four-way switch, which is used to make a "suicided" 3-way switch. The power comes in AND goes out to the light from that switch. The other switch is a 3-way. Neither one has the "ON" or "OFF" marks on them, do they? You can buy a 3-way dimmer, but i do not know if rotary-style 3-way switches exist. You will have to install the dimmer at the other switch because i am quite certain no 4-way dimmers are available.

SOURCE: Replace 3 way dimmer but different color wires how to connect?

you will want to make the following connections
existing-black wall to black dimmer now connects to red dimmer
existing black wall to red dimmer now connects to black dimmer
existing red wall to black dimmer now connects to red dimmer
Green wire connects to bare copper wires in wall

SOURCE: We have a leviton dimmer

HI,
Usually there is only a red wire when there are 2 switches for the same light (a 3way) if that is the case you need a switch that is a 3 way, there are some 3 way dimmers. But you do need to replace both switches.
The other thing may be at the light fiture end. If the red wire is in there it should be connected to a black wire, which should only be connected to each other, that way the curcuit is broken.
or to say one white wire is connect to the light, a black wire connected to the red,(taking power to the switch) a black wire is returning from the light switch is connected to the light. (if it is a 3 way its different...)
Confused... 1st confirm if its a 3 way, second confirm if there is a red wire at the light fixture, from there I can guide you more.

Brett