Since you're using the plain bibliography style, the field
author= { Kotidis , Faloutsos ,Labrinidis},
is interpreted as featuring a single author with surname-component "Kotidis", junior-component "Faloutsos", and first-name-component "Labrinidis". Note that commas in the author field do not separate authors; instead, they only serve to separate various components of a single (composite) name. In the formatted output, you therefore get
Labrinidis Kotidis, Faloutsos
Observe that the assumed junior component -- "Faloutsos" -- is separated with a comma from the assumed firstname and surname components.
What's going on?! There's both a syntactic mistake and a content mistake in your author field. The syntax mistake is that you're using commas instead of the keyword and to separate the authors. The content mistake is that you've failed to provide the first names for the three authors. You really ought to write the field as either
author = "Yannis Kotidis and Christos Faloutsos and Alexandros Labrinidis",
or
author = "Kotidis, Yannis and Faloutsos, Christos and Labrinidis, Alexandros",
With either form, BibTeX will parse the field correctly, and the bibliography style is left determine if full or abbreviated first names should be shown. The plain bibliography style is programmed to show the full first names, by the way.