a = {{{-17.8227277373099`, -1.6565234964560602`},
{-1.6565234954649242`,
 5.298073701591974`}}, {{-17.812203521929003`,
-1.5013126607114478`}, {-1.5013126574896714`,
 4.384050851253119`}}, {{-17.801677045750512`,
 -1.4055541329078751`}, {-1.405554138172727`,
 3.869511752542245`}}};

b = {{{0.8409518416651456`, 0}, {0,
 0.1274293000222242`}}, {{0.8409815693580924`, 0}, {0,
 0.14187218616724442`}}, {{0.841011296000238`, 0}, {0,
 0.15290209433231844`}}};

mat = b.a.b

a1 = {{-17.8227277373099`, -1.6565234964560602`},
{-1.6565234954649242`, 5.298073701591974`}};

b1 = {{0.8409518416651456`, 0}, {0, 0.1274293000222242`}};

 mat1 = b1.a1.b1


 {{-12.6042, -0.177516}, {-0.177516, 0.0860313}}

Thanks.