  We report on the spectral features of the Si IV 1402.77 \AA, C II 1334.53
\AA, and Mg II h or k lines, formed in the layers from the transition region to
the chromosphere, in three two-ribbon flares (with X-, M-, and C-class)
observed with IRIS. All the three lines show significant redshifts within the
main flare ribbons, which mainly originate from the chromospheric condensation
during the flares. The average redshift velocities of the Si IV line within the
main ribbons are 56.6, 25.6, and 10.5 km s$^{-1}$ for the X-, M-, and C-class
flares, respectively, which show a decreasing tendency with the flare class.
The C II and Mg II lines show a similar tendency but with smaller velocities
compared to the Si IV line. Additionally, the Mg II h or k line shows a
blue-wing enhancement in the three flares in particular at the flare ribbon
fronts, which is supposed to be caused by an upflow in the upper chromosphere
due to the heating of the atmosphere. Moreover, the Mg II h or k line exhibits
a central reversal at the flare ribbons, but turns to pure emission shortly
after 1--4 minutes. Correspondingly, the C II line also shows a central
reversal but in a smaller region. However, for the Si IV line, the central
reversal is only found in the X-class flare, but not in the other two flares.
As usual, the central reversal of these lines can be caused by the opacity
effect. This implies that in addition to the optically thick lines (C II and Mg
II lines), the Si IV line can become optically thick in a strong flare, which
is likely related to the nonthermal electron beam heating.
