Sounds like your caliper is a bit sticky .
Often , if the pads are worn the exposed portion of the piston can get dirtied up making it reluctant to return when you release the brake ! The same can happen after having new pads fitted .
Try removing the caliper and putting the gentlest touch on the brake pedal , just enough to get the piston past their sitting point . Then remove the pads and give the caliper a good clean up with brake cleaner concentrating on the pistons . When spanky clean , a thin coat of coppaslip brake grease around the exposed parts of the piston and seals , a very thin smear to the back of the pads and refit them to the calipre . Ease the pads apart with a blunt instument such as a thin tyre iron and refit the caliper to the disc !
But obviously only do this if pedal adjustment etc is good ! The disc would need to be badly scored , cracked between mounting holes or run so hot it had warped before it would need replacing . If the disc was warped you would more than likely feel that as pulsing in the pedal or as a grab when you pushed the bike along . If in doubt , lift the rear wheel and check the runout of the disc !
Remember tho , the r3s are heavy beasts and the discs do get hot under braking but rarely hot enough to warp on a rear !