A triple crank will probably require a different bottom bracket with a longer spindle. A modern sealed bearing square taper bottom bracket is not expensive. The crank itself can be bought used. Either a road triple or a mountain triple can be used, with the road more likely to look "right" on your classic Raleigh. Most likely your front derailleur will reach far enough, and that can be checked before making the swap. You will not need a longer chain. Your existing rear derailleur can certainly accommodate a 26 or 28 low cog.

The main advantage of the triple is that you can use a smaller "low" cog on the freewheel for smaller steps between the gears, and avoid the look of a pie plate cog if that matters to you. 30 x 26 is lower than 42 x 34. 30 x 28 is lower yet, and a 14-26 and 14-28 six speed freewheels are both pretty common. Or, if you use a mountain triple, you can get an even lower gear, 22 x 28 for example. The main disadvantage is that the conversion is a little more involved.