Concrete – Is this joint between the foundation and first row of mortar typical

Three inch gaps seem quite large - even for a 300 year old wall.

You just need to use a good mortar mix (sand and cement) on the narrower gaps.

Clean out all the old, lose material first to make sure that you get good adhesion and then push the mortar right into the gaps as far as it will go. Work on a small area (1 m²) at a time.

Adding pea gravel to give the mix more "body" might work on the wider ones.

However, I'd get a professional to have a look at the wall first to make sure you're not going to have to redo this work in a couple of years time. If the foundations have gone it might need something more substantial work doing.

Is the wall part of a house or just a garden wall? If it's the latter then dismantling and rebuilding might be the best long term option.

Quikrete FASTSET Repair Mortar is a great product. The biggest difference between it and a typical type S mortar is that it is "fastset" (duh) meaning it will be hard in about 20 min. Not cured, but hardened. And that it is intended for commercial use as it will eventually reach 6000 psi after it's full 28 day cure. This product does not need any bonding agent as it is already added. http://www.quikrete.com/ProductLines/FastSetRepairMortar.asp

Any regular type S mortar will harden in about a day or two depending on the enviorment. And will eventually reach 1800psi http://www.quikrete.com/ProductLines/MasonMix.asp

You may think "oooo stronger is better" but not so with mortar. You always want the mortar to be weaker then the surrounding material (stone, brick, cinder block) That is why it comes in so many strengths. From strongest to weakest type, M S N O K.

Sorry to be so long winded.

Basically, go with the type S, and use the FastSet Repair mortar for repairs around the house.