The adjective virtual here can be thought of as approximately.
Some definitions from the Wordnik page include:
Existing or resulting in essence or effect though not in actual fact
In effect or essence, if not in fact or reality
So the main idea is this: There is no need to debate which of the three stocks was actually the worst; all three of them performed so badly that we might as well clump them together and call it a three-way tie, without spending the time to calculate which one was actually the worst.
As another example, suppose I gave an exam, and the 18 students in my class scored as follows:
94 92 90 90 89 88 87 87 86 84 77 76 76 74 72 52 51 50
I might tell the student who scored the 51: You were in a virtual three-way tie for the lowest score in the class. This isn't precisely factual, because this student in fact scored one point higher than one student, and one point lower than another. But the three scores are so close together and so much lower than the rest of the class that I can call it a virtual tie. It's essentially a three-way tie, even though it's not technically a three-way tie.