if you are using some round robin machanism then in between your running loop you can just poll the pin connected to the switch and if that is pressed(i,e 0)increase a counter variable by 1 and after this check if this overflow a fix value X,if yes then increment the display count.if the pin is high then simply reset the counter varriable.the fix value X is to be choosen by some hit-trial,and that also depends on the length of round robin loop.

>>
main()
while(1) {
//........some task code
//........some task code
//........some task code
if(button1 == 0){
count_var++; }else{
count_var = 0; }
if(count_var1 > scan_delay) {
// then increment the count value }
//........some task code

if(button2 == 0){
count_var++; }else{
count_var = 0; }
if(count_var2 > scan_delay) {
// then increment the count value }

//........some task code

if(button3 == 0){
count_var++; }else{
count_var = 0; }
if(count_var3 > scan_delay) {
// then increment the count value }

//........some task code
//........some task code


}
in above code you will have to tweak the value of scan_delay constant as per human behaviour so that even a single key press can be detected...actually i tried above scheme in one of my project and it worked well .
note>> for getting more precision try putting the above code inside a timer interrupt handler configured to fire at 500 microseconds.