Origin of broad CO stretch in infrared spectrum of tricobalt cluster
I found an reliable article (http://www.ilpi.com/organomet/carbonyl.html) describing wide spectrum of metal carbonyls. Since Chemistry SE won't open that website, I have attached scheme with a range of carbonyl stretching frequencies in the article for your benefits: 



The IR stretching of carbonyl can be varied based on the structure. According to the website:


  In the IR, typical stretching frequencies are: (i) Uncoordinated or "free" $\ce{CO}$: $\pu{2143 cm^{-1}}$; (ii) Terminal $\ce{M-CO}$: $\pu{2125}$ to $\pu{1850cm^{-1}}$; (iii) Doubly bridging ($\mu_2$): $\pu{1850}$ to $\pu{1750 cm^{-1}}$; (iv) Triply bridging ($\mu_3$): $\pu{1675}$ to $\pu{1600 cm^{-1}}$; and (v) Semibridging: somewhere between terminal and $\mu_2$.


In literature, most tricobalt clusters were discussed included with other metal cations. For example: (a) Rational synthesis of tricobalt-molybdenum and -tungsten butterfly clusters with alkyne ligands in Journal of Organometallic Chemistry, 2002, 659(1-2), 142-150 (https://doi.org/10.1016/S0022-328X(02)01720-5) and (b) Clusters as Ligands. 5. Tricobalt Cluster Alkoxycarboxylates of Titanium and Zirconium Exhibiting Novel Structures and Properties in Organometallics, 1997, 16(24), 5289–5301 (https://pubs.acs.org/doi/abs/10.1021/om970837b). 

You can also read: A. S. Goldman, K. Krogh-Jespersen, Why Do Cationic Carbon Monoxide Complexes Have High $\ce{C-O}$ Stretching Force Constants and Short $\ce{C-O}$ Bonds? Electrostatic Effects, Not $\sigma$-Bonding, J. Am. Chem. Soc., 1996, 118, 12159-12166 (https://cdn-pubs.acs.org/doi/10.1021/ja960876z).

Since you have completely described preparation procedure of your tricobalt cluster, I assume the broasness of carbonyl absorptions are due to different structural aspects of carbonyl on $\ce{Co}$ and other metals in the cluster. It is also possibility that you have a mixture of clusters. Note that range of $\nu_\mathrm{CO}$ could be $\pu{2100}$ to $\pu{1300 cm^{-1}}$, based on stereo arrangement (see above references).