nevada1.rb

require 'benchmark/ips'

Benchmark.ips do |ips|
  N = 1326767

  ips.report('a.join.reverse.to_i') { N.digits.each_slice(3).map { |a| a.join.reverse.to_i } }
  ips.report('step(3).map') { (0...N.to_s.size).step(3).map{ |i| (N/10**i) % 1000 } }
  ips.report('Math.log10') { N.then { |n| 0.step(by: 3, to: Math.log10(n.abs).floor.succ).map { |i| n / 10 ** i % 1000 } } }
  ips.report('|a, b = 0, c = 0|') { N.digits.each_slice(3).map { |a, b = 0, c = 0| a + b * 10 + c * 100 } }

  ips.compare!
end

results.txt

Warming up --------------------------------------
 a.join.reverse.to_i    21.720k i/100ms
         step(3).map    37.398k i/100ms
          Math.log10    32.586k i/100ms
   |a, b = 0, c = 0|    49.818k i/100ms
Calculating -------------------------------------
 a.join.reverse.to_i    226.055k (± 4.8%) i/s -      1.129M in   5.007341s
         step(3).map    467.119k (± 6.4%) i/s -      2.356M in   5.065829s
          Math.log10    369.276k (± 7.7%) i/s -      1.857M in   5.062217s
   |a, b = 0, c = 0|    675.102k (± 8.6%) i/s -      3.388M in   5.062249s

Comparison:
   |a, b = 0, c = 0|:   675101.6 i/s
         step(3).map:   467118.9 i/s - 1.45x  slower
          Math.log10:   369275.6 i/s - 1.83x  slower
 a.join.reverse.to_i:   226055.1 i/s - 2.99x  slower