How to you show a 3-center 2-electron bond in a Lewis structure?
What about the probably "not so good" approach might be the bent bond as sometimes seen for diborane?



(source)

The latest recommendation by IUPAC I know, exemplified in rule "GR-1.9 Multi-center bonds":



(Brecher, Pure Appl. Chem., 80, 277-410, 2008; doi 10.1351/pac200880020277, open access)

stating:


  "As a matter of convention, any such multi-center character is ignored
  when producing chemical structure diagrams, and regular bonds
  connecting pairs of atoms are used instead."




To accommodate Karsten Theis' comment on the initial form of this answer, I would like to add the perspective provided by Robert Grossmann's The Art of Writing Reasonable Organic Reaction Mechanisms.  Starting page 270 (2nd edition, 2003) the chapter introducing reactions with transition metals draws attention about "Conventions of Drawing Structures"; highlighting that there are field dependent conventions about what line and dash represent.  To quote:


  "The conventions for drawing organometallic and inorganic compounds
  differ in subtle ways from those used to draw “ordinary” organic
  compounds. The most important difference is the way in which bonds are
  drawn. In organic compounds, one does not use a line to connect a bond
  to an atom. In organometallic and inorganic compounds, however, a line
  sometimes connects an atom and a  $\sigma$ or $\pi$ bond. In this
  case, the line indicates that the pair of electrons in the $\sigma$ or
  $\pi$ bond is shared with the metal also."



(loc. cit. p. 271)


  "An even more confusing situation arises in complexes in which the
  electrons in a $\pi$ system spread over three or more atoms are used
  to make a bond to a metal. In this case, the usual convention is to
  use a curved line to indicate the $\pi$ system and a single line to
  connect the $\pi$ system to the metal, regardless of the number of
  electrons in the $\pi$ system (the organometallic chemists’
  convention). However, sometimes the curved line is omitted and single
  lines are used to connect the metal to each of the atoms in the $\pi$
  system (the crystallographers’ convention). The representation that
  would make the most sense to organic chemists, in which a single line
  represents a two-electron  bond and a dative bond is used to show
  two-electron bonds between each individual $\ce{C=C}$ $\pi$ bond and
  the metal, is simply never used."



(loc. cit., p. 271)


  "Formal charges are usually omitted in inorganic and organometallic
  complexes. Only the overall charge on the complex is indicated. For
  example, formal charges are usually not assigned in Lewis acid–base
  complexes involving transition metals. The acid–base bond is sometimes
  indicated by an arrow pointing from the ligand to the metal, but more
  often it is indicated by an ordinary line."



(loc. cit., p. 271)

This precedes just the section about electron counting (specifically in organometallic compounds with transition metals). I speculate, however, Robert Grossman aiming to highlight differences between "organic" and "organometallic" convention meant does while accidentally deploying does not in the first paragraph.