Strong acids are:
HCl, H2SO4, HNO3
Any acids other than those are Weak acid
H3PO4 is said to be triprotic although it is Weak acid
Indicator:
Stong acid + Stong base <---Use bromothymol blue
Stong acid + Weak base <---Use methyl orange
Weak acid + Stong base <---Use phenolphthalein