BENGALURU, Dec 23 (Reuters) - Indian shares gained for a third straight session on Thursday, led by metals and banking stocks, as risk sentiment improved globally after a study showed hospital stay risk for the Omicron COVID-19 variant is lower than the Delta strain. The NSE Nifty 50 index (.NSEI) rose 0.56% to 17,054 by 0345 GMT and the benchmark S&P BSE Sensex (.BSESN) gained 0.55% to 57,242.97. The risk of needing to stay in hospital for patients with the Omicron variant is 40% to 45% lower than for patients with the Delta variant, according to research by London"s Imperial College published on Wednesday. read more The Nifty bank index (.NSEBANK) rose 0.7%, while the metals index (.NIFTYMET) was up 1%.