Can Hydronium Act as a Ligand?
This is not the answer OP would expect because I couldn't find metal complexes of type $\ce{[M(H3O)_x]^{y+}}$ (the reason for non-existence(?) is mentioned in the comments) but I did find a complex of an organic compound where hydronium ion act as a ligand i.e a complex of 18-crown-6 and hydronium ion(emphasis mine):

The observation that cyclic crown ethers can bind cations with high
affinity and selectivity has had a tremendous impact in chemistry. The
complex between 18-crown-6 (18c6) and the hydronium ion, $\ce{H3O+}$,
is a prototypical representative of this important host−guest paradigm
and has been the subject of numerous experimental and theoretical
studies. The computational evidence presented so far has favored a
structure with three linear $\ce{O−H···O}$ hydrogen bridges as the
lowest, stable minimum, whereas a bifurcated arrangement has turned
out to be a transition state for $\ce{H3O+}$ rotation 4−5 kcal/mol
higher in energy.


The closest metal complex I could find where hydronium ion is present as an entity is hydronium zinc(II) pyrazine-2,3-dicarboxylate(again not the type of complex OP is looking) where 2,3-PZDC (μ-pyrazine-2,3-dicarboxylato-N,O,O′) act as ligand(emphasis mine):

The structure of dihydronium
[catena-bis(μ-pyrazine-2,3-dicarboxylato-N,O,O′)zinc(II)],
$\ce{(H3O)2[Zn(2,3PZDC)2]}$, is composed of polyanionic ribbons of
zinc(II) ions linked by double bridging 2,3-PZDC ligand molecules.
Each ligand uses an N,O bonding moiety formed by one carboxylic group
[$\ce{Zn–O}$ 2.071(2) Å; $\ce{Zn–N}$ 2.184(2) Å] and a monodentate oxygen atom of
the other carboxylate group [$\ce{Zn–O}$ 2.092(2) Å]. Coordination around the
zinc(II) ion is strongly distorted octahedral. Hydronium cations
$\ce{H3O+}$ link the ribbons by hydrogen bonds.

References

Hydronium Ion Complex of 18-Crown-6:  Theory Confirms Three “Normal” Linear Hydrogen Bonds
Michael Bühl, Ralf Ludwig, Rachel Schurhammer, and, and Georges Wipff,
The Journal of Physical Chemistry A 2004 108 (51), 11463-11468
DOI: 10.1021/jp045879+
Michal Gryz, Wojciech Starosta &amp; Janusz Leciejewicz (2005) &quot;Doubly bridged molecular ribbons in the structure of an ionic complex, hydronium zinc(II) pyrazine-2,3-dicarboxylate&quot;, Journal of Coordination Chemistry, 58:11, 931-935, DOI: 10.1080/00958970500055492