Desea usar flex en .btn-group y hacer que tome el 100% del ancho del padre para estirar el div.

 .btn-group { width: 100%; display: flex; justify-content: center; background-color: skyblue; } .btn-group button { border: 3px solid green; padding: 10px; background-color: #04AA6D; /* Green background */ border: 1px solid green; /* Green border */ color: white; /* White text */ padding: 10px 24px; /* Some padding */ cursor: pointer; /* Pointer/hand icon */ } .btn-group button:not(:last-child) { border-right: none; /* Prevent double borders */ } /* Clear floats (clearfix hack) */ .btn-group:after { content: ""; clear: both; display: table; } /* Add a background color on hover */ .btn-group button:hover { background-color: #3e8e41; }

 <div class="btn-group"> <button type="button" class="button1">Faucet</button> <button type="button" class="button2">About Nano/Faucet</button> <button type="button" class="button3">Donate</button> </div>

Puedes agregar a tu css:

 .btn-group{ display:flex; justify-content:center; }

se verá así

Puedes usar flex box

 .btn-group { display:flex; flex-direction: row; justify-content:center; } .btn-group button { border: 3px solid green; padding: 10px; background-color: #04AA6D; /* Green background */ border: 1px solid green; /* Green border */ color: white; /* White text */ padding: 10px 24px; /* Some padding */ cursor: pointer; /* Pointer/hand icon */ } .btn-group button:not(:last-child) { border-right: none; /* Prevent double borders */ } /* Clear floats (clearfix hack) */ .btn-group:after { content: ""; clear: both; display: table; } /* Add a background color on hover */ .btn-group button:hover { background-color: #3e8e41; }

 <div class="btn-group"> <button type="button" class="button1">Faucet</button> <button type="button" class="button2">About Nano/Faucet</button> <button type="button" class="button3">Donate</button> </div>