Lightweight Natasha Jonas and flyweight Nicola Adams are both assured a place on Team GB after reaching the last four in their categories.
The top three European boxers in each weight division this week achieve the selection standard but Marshall, after beating China's Li Jinzi 20-8, advanced to a final four dominated by European rivals.
That means the 2010 silver medallist must win again to make totally sure of her place or she will be reliant on a meeting of an independent commission next month.
However, there was no hiding the delight for British Boxing's performance director Rob McCracken, who will now take a minimum of nine boxers to the Games, including seven men who booked spots at last year's World Championship and the recent Olympic qualifier in Turkey.
He said "To have secured two Olympic places and be guaranteed three medals is a fantastic achievement.
"All three of the women boxed fantastically and I am delighted for Natasha and Nicola.
"Savannah has boxed superbly this week in an extremely tough draw.
"The aim now is to ensure that everyone stays focused and that we do all we can to ensure Savannah qualifies and that all of the boxers go as far as they can in this tournament."
Adams, who won the European title earlier this year, beat a five-time world light flyweight champion, India's Mery Kom, to progress to the semi-finals.
"That was such a tense contest against one of the legends of women's boxingm so to have beaten Mery Kom and secure a place in the Olympics at the same time is absolutely brilliant," she said.
"This win has given me a huge confidence boost and the challenge now is to keep my concentration and ensure I go as far as I can here."
Jonas will face a tough challenge though to improve on her bronze with Russia's Sofya Ochigava her next opponent and Ireland's Katie Taylor, the three-time world and five-times European champion, lurking on the other side of the draw.
"To know that I'm going to be going out in front of 10,000 people and they're all going to be cheering for me for a change is just unreal," said Jonas.
"I just can't wait to get onto that stage and show everybody what women's boxing is all about.
"It's has been such a tough journey to get to this point and to have finally qualified is the most amazing feeling.
"To compete in the Olympic Games is my dream and I am just so relieved to have secured both my place at London and a medal in these Championships."