There seems to a be a mixup in your post where you have mixed up bond funds with equity funds.
VTSMX -> FSTVX, check
VGTSX -> FSGDX (this you got misxed up!) FSGDX does have emerging markets equities along with developed market equities. These funds follow the same index.
VBMFX -> FXSTX check. VBMFX does not have foreign bonds. It is a US bond index fund just like FXSTX.
You do not need to venture away from those 3 Fidelity funds if you want the equivalent of a 3 fund Vanguard portfolio.
It's all about market timing, uh, I mean rebalancing, uh, I mean opportunistic rebalancing, uh, I mean short-term opportunistic rebalancing due to a short-term change in one's asset allocation.