You can give this a try, however this alternative is with four borders, not the three. With RGBA you can change the opacity. You can visit http://www.cssportal.com/css3-rgba-generator/ to generate the CSS3 RGBA colours; there you can also change the opacity.
div {
width: 100px;
height: 100px;
margin: auto;
}
.one {
border-top: 1px solid rgba(0, 255, 0, 0.7);
border-right: 1px solid rgba(255, 0, 0, 0.7);
border-bottom: 1px solid rgba(0, 0, 255, 0.7);
border-left: 1px solid rgba(255, 0, 0, .5);
<div class="one"></div>
Alternatively if you wanted to go for more of a gradient look you can try applying a CSS3 gradient within a pseudo-element, however only two border colors are adopted, and it's without the opacity.
.one{
margin: auto;
width: 100px;
height: 100px;
border: 1px solid transparent;
-moz-border-image: -moz-linear-gradient(top, #E93478 0%, #FF0 100%);
-webkit-border-image: -webkit-linear-gradient(top, #E93478 0%, #FF0 100%);
border-image: linear-gradient(to bottom, #E93478 0%, #FF0 100%);
border-image-slice: 1;}
<div class="one"></div>