Enclose the button in <div>'s, and tweak the margins so that they look right and line up correctly. Say you want the images to be 10% of the screen. Since you have 3 buttons, 2 buttons can take up 30%, and one button can take up 40%. So you want to set up your margins to look something like this.
20% margin-left Paypal button (10%) | 15% margin-left Paypal button (10%) 15% margin-right | Paypal button (10%) 20% margin-right
and if you have your doctype correct,
HTML Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
It should work close to the same way for every browser.
Good luck!