There are some "flat panel" speakers (like the Yamaha you mention) that are meant to be used L/C/R and whose width dimension is around 5". Some of these speakers can be quite good - look at the ones made by Paradigm. Look at the used market if budget is a real problem.
The main downside to MTM (mid - tweeter - mid) LCR speakers (all 3 front identical) is the horizontal dispersion of the sound - especially for the center speaker. For the L/R, the tweeter and the woofers are all lined up vertically so the sound from the different drivers is aligned. When you put that same speaker on its side and sit anywhere in the room other than directly in front of the speaker, the sound from the different drivers arrives at your ears at different times. This is called "lobing" and can cause dialog to be hard to understand - but I had speakers like this for 10+ years and were happy with them.
A dedicated center speaker with a "stacked" tweeter / midrange flanked by the woofers will have superior dialog intelligibility, but for a living room system you will probably be more than happy with the LCR type speaker in the center. Whether you use that same speaker for the L/R is personal preference and whether you have money and space for larger main speakers.
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD