What is the difference between the 3 Singapore banks? Which one should I invest in?

Sama sama in their capitalization since all under the watchful eye of MAS, a real distinction could be made in their exposure to China/Hongkong loan books

Depends on what you are looking to achieve.

For dividends (before COVID-19):
DBS and UOB have been paying $1+/share for the financial year recently. DBS is quarterly payout while UOB is semi-annual. OCBC’s dividend is around $0.60/share.

For valuation:
OCBC should have the lowest P/B ratio of the three now. DBS should have the highest P/B.

For price:
OCBC should be the cheapest of the three. UOB and DBS are almost the same level.

There are also other factors like their growth strategy and regional footprint, technology usage etc.