The vertical asymptote can be calculated with the definition of limit:
Reduce[ForAll[n,
Exists[\[Delta], \[Delta] > 0,
ForAll[x, 0 < Abs[x - x0] < \[Delta],
Abs[(x^2 + x + 3)/(x - 1)] > n]]],
x0, Reals]
The diagonal one with Series
:
Series[(x^2 + x + 3)/(x - 1),
{x, Infinity, 0}]