Can you put triple glazing in existing frames?

Gloved hands lifting a triple glazed sealed unit into an existing white window frame

Usually not usefully. Most UK double glazed frames take sealed units up to about 28 mm thick, while a good triple unit is 36 to 44 mm. A 28 mm triple unit that fits reaches only about Ug 1.2, worse than good double glazing, and the extra weight can strain the hinges.

Why the rebate depth matters

The rebate is the slot in the frame the glass sits in. Typical double units are 24 to 28 mm. To fit a thicker triple unit you need a frame designed for it, with deeper beads and drainage, and hinges rated for about 50% more weight.

The thin-triple trap

A triple unit squeezed into 28 mm has two narrow cavities. Narrow cavities insulate poorly; BRE gives about 1.2 W/m²K for a thin 4-8-4-8-4 unit with one low-e coating, against about 1.1 for a good 4-20-4 double unit. You pay more for a worse result.

When it does work

Frames designed for triple glazing (common on newer European systems and low-energy builds)
Replacing a failed triple unit like for like
Some fixed panes in deep frames

The better alternative

If your frames are sound but the units have failed, replace them with the best double units the frame can take: warm-edge, argon filled, low-e. If the frames are tired, replace the whole window with a triple glazed one.

Questions

Can I just replace the glass with triple?

Only if the frame has a deep enough rebate and hinges rated for the weight. Most standard double glazed frames do not.

Is a thin triple unit better than double?

Usually not: a 28 mm triple unit performs about the same as, or worse than, a good double unit.

Sources

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