I admire vegetarianism (I’m not one yet, but at this point eat no meat most days). However, I think there’s a certain harm quotient or “kill count” associated with the production of almost all foods.
For example, for cereal crops like maize or soy or wheat, the land needs to be prepared and ploughed (killing insects and worms), defended from pests like insects, snails, rodents, small animals, birds (even with organic methods, e.g. introducing biological predators or cats for rodents, that involves some killing), and then harvesting with a certain kill count also (insects, small nesting birds, small animals etc.).
Raising cattle and other animals by using cereal crops as feed makes things far worse (inefficiently converting plant calories to meat calories – for beef IIRC the ratio is about 30 – and adding the extra animal suffering and death on top of the magnified crop kill count).
Things are a bit greyer for dairy raised on natural pasture (as happens in Ireland where the weather is mild enough that dairy cattle feed on grass for almost all of the year). Pesticides do not need to be used and there doesn’t have to be ploughing and harvesting (assuming grass isn’t cut for silage). There are some other cruelties involved in the process though (male calves can’t produce milk and often end up raised as bullocks for beef), calves are separated from their mothers, many modern breeds are inhumanely too optimized to produce large quantities of milk and cows are generally slaughtered at around 5 years old (could naturally live to around 20). Some of those cruelties could be reduced at some cost to efficiency (use less modern breeds, use sexed semen in breeding to mostly eliminate male calves, and at the cost of about a third of the milk allow calves to suckle for a few months). All that would probably drive up by two to three fold the cost of the milk. However, allowing cows to live 20 years instead of 5 would probably triple the cost again (AFAIA some dairies run by Hare Krisha in the UK do this). A single cow living for 5 years on grass usually produces more than enough milk for a single lacto vegetarian in their lifetime. That’s the associated kill count (a lot better than the multiple cows a typical human beef eater will consume over a lifetime I guess).
Something similar holds if you feed dairy cows straw from cereal crops or unused byproducts of plant crops. In terms of efficiency, the most optimal farming systems are mostly plant based with a small amount of animal rearing that uses up food waste and residues and byproducts like straw of cereal crops.
Producing eggs is possibly more easily made ethical than dairy. Egg sexing technology is becoming quite viable and starting to be used commercially (ensuring no male chicks are produced and discarded). If heritage breeds are used and living conditions are good and chickens allowed to live their full lifespan, it’s IMO pretty ethical. I actually have a workmate doing this in his back garden (using rescued battery hens). Usually, in commercial batteries, hens are killed after 2 or 3 years when egg production falls a bit. However, leaving hens live a natural lifespans isn’t as inefficient as for cattle (generally, it gradually trails off over years, with 7 or 8 year old hens often still producing something, just infrequently). Probably just doubles the inefficiency.
Meat does take a lot of land to produce whether directly via grazing or indirectly feeding animals cereal crops. Vegetarian diets (or mostly vegetarian diets) need a lot less land (all a bit pointless if left-over land is merely used to grow crops for biofuels). Potentially, if efficient enough, we could rewild some of the countryside. An interesting question is would that actually reduce suffering? Wild land would involve wild animals with some predators and more prey (deer and wolves or wildebeests and big cats etc.). Wild animals generally tend to live stressed and shorter lives (zoo animals almost always live much longer) with predators eventually starving when they can no longer hunt or prey animals ending their lives devoured by a predator or bunch of them (there are rare exceptions such as big animals like elephants with no natural predators). Life in the wild is no picnic. So, paradoxically, there may be more animal suffering going on in an area of wild land than cultivated land. In relative term, the life of a cow on pasture (if looked after humanely) probably is in many ways better (protected from predators, food and water easily available) than that of a wildebeest (death is more humane too).
So comparing the relative amount of suffering going on in the same area of cultivated cereal, farmed pastureland system or equivalent area of wild savannah/prairie would be interesting (am not sure if anyone has ever tried to do it in a scientific quantitative sense). I suspect option 3 would be worst. However, am not so sure how options 1 and 2 would fare in relative terms.