Mr H2co3

A protonated carbonic acid (trihydroxymethyl cation)
$\ce{H3CO3+}$
does be, and is in fact incredibly stable in the solutions of superacids [1] upwardly to
$\pu{0 °C}.$
One way of obtaining
$\ce{H3CO3+}$
is dissolving inorganic carbonates and hydrogen carbonates in magic acid at
$\pu{-80 °C}$
[ii]:

$$\ce{CO3^ii- or HCO3- ->[FSO3H-SbF5/SO2][\pu{-80 °C}] H3CO3+ ->[\pu{-x – 0 °C}] CO2 + H3O+}$$

As for the construction, the
$C_\mathrm{3h}$-symmetrical molecule (triskelion shape, construction
7
beneath) was establish to be the global minimum [2] (reference numbers updated):

We too carried out ab initio and IGLO calculations […] on bicarbonate besides as free and mono-, and diprotonated carbonic acids to rationalize the observed experimental results. Initially the possible geometries were optimized at the HF/half-dozen-31G* level. The most stable isomers were afterwards adamant from relative energies at the MP2/half-dozen-31G*//HF/6-31G* level. Finally, the lowest energy structures were further optimized at the MP2/6-31G* level. MP2/six-31G* optimized structures were used to calculate
$\ce{^{13}C}$
NMR chemical shifts.

[…]

Protonated Carbonic Acid. The symmetrical
$C_\mathrm{3h}$
structure
7
was establish to be the global minimum. This structure was suggested for the observed species in superacid solution by Olah et al. [ane] on the ground of
$\ce{^1H}$
and
$\ce{^{13}C}$
NMR spectroscopy. The ion at
$\pu{-eighty °C}$
shows single sharp peaks at
$δ(\ce{^1H})~12.05$
and
$δ(\ce{^{xiii}C})~165.4$
in the proton and
$\ce{^{thirteen}C}$
NMR spectra, respectively.

Protonation of carbonic acid on the hydroxyl oxygen atom (8) was shown to exist unfavorable by
$\pu{23.four kcal/mol}$
over protonation on the carbonyl oxygen (seven). Frequency calculation at the HF/6-31G*//HF/six-31G* level showed that the structure
8
is non a minimum, equally it contains two imaginary frequencies.

$\ce{H3CO3+}$
shares structural similarities with its triaza-analog, the guanidinium ion, as both are possessing resonance stabilization via their onium forms [3, p. threescore].

References

1. Olah, G. A.; White, A. M. Stable Carbonium Ions. LXIV. Protonated Carbonic Acrid (Trihydroxycarbonium Ion) and Protonated Alkyl (Aryl) Carbonates and Hydrogen Carbonates, and Their Cleavage to Protonated Carbonic Acrid and Carbonium Ions. The Possible Part of Protonated Carbonic Acid in Biological Carboxylation Processes.
   J. Am. Chem. Soc.
   1968, 90 (seven), 1884–1889. https://doi.org/10.1021/ja01009a036.
2. Rasul, G.; Reddy, 5. P.; Zdunek, L. Z.; Prakash, 1000. K. S.; Olah, G. A. Chemical science in Superacids. 12. Carbonic Acrid and Its Mono- and Diprotonation: NMR, Ab Initio, and IGLO Investigation.
   J. Am. Chem. Soc.
   1993, 115 (half dozen), 2236–2238. https://doi.org/10.1021/ja00059a020.
3. Dewar, M. J. S., Hafner, K., Heilbronner, East., Itô, S., Lehn, J.-M., Niedenzu, K., Rees, C. Westward., Schäfer, One thousand., Wittig, G., Boschke, F. Fifty., Series Eds.;
   Topics in Current Chemical science; Springer Berlin Heidelberg: Berlin, Heidelberg,
   1979; Vol. 80.