Get her alone. Create a more intimate environment between you and your best friend away from your other friends. By spending more one-on-one time, she may get the idea that you are interested without you having to ask her directly. This may also offer you the opportunity to change your mind without breaking the friendship if she shows a lack of interest. Begin with little things, like hiking, dinner for the two of you or simply coffee and great conversation.
Find out if he likes to be with you. Take things one step further if your best friend seems to enjoy your one-on-one time together. Because you are best friends, you have the advantage of asking him out without having to label it as a date just yet. Ask him if he would like to go out dancing, a great way to get physically closer and begin to test your ground.
Assess the situation. After so much alone time and a few nights out getting physically close, you should now know whether your best friend is interested in something more. If not, you may need to back off and keep things as they are, as you don't want to lose the friendship. However, if you think she may just want to date you, it's time to make a move.
Make physical contact. Go bold and find a romantic moment to hold his hand or even move in for a little kiss. If you are not comfortable with such drastic attempts, speak directly to your best friend. Tell him how much you have enjoyed your alone time, and that you would love to begin going on regular dates.